ci: always run fixed 3-config matrix in nix workflow

Drop full_matrix/minimal_matrix selection logic (branch, PR title,
commit message, labels, workflow_dispatch input) and always build
gcc-13, clang-20 llvm-cov coverage, and clang-16 libstdc++.

.github/workflows/xahau-ga-nix.yml

@@ -8,12 +8,6 @@ on:
     types: [opened, synchronize, reopened, labeled, unlabeled]
   schedule:
     - cron: '0 0 * * *'
-  workflow_dispatch:
-    inputs:
-      full_matrix:
-        description: "Force full matrix (6 configs)"
-        required: false
-        default: "false"
 
 concurrency:
   group: ${{ github.workflow }}-${{ github.ref }}
@@ -30,37 +24,14 @@ jobs:
     outputs:
       matrix: ${{ steps.set-matrix.outputs.matrix }}
     steps:
-      - name: escape double quotes
-        id: escape
-        shell: bash
-        env:
-          PR_TITLE: ${{ github.event.pull_request.title }}
-        run: |
-          ESCAPED_PR_TITLE="${PR_TITLE//\"/\\\"}"
-          echo "title=${ESCAPED_PR_TITLE}" >> "$GITHUB_OUTPUT"
       - name: Generate build matrix
         id: set-matrix
         shell: python
-        env:
-          GH_TOKEN: ${{ github.token }}
         run: |
           import json
           import os
-          import urllib.request
 
-          # Full matrix with all 6 compiler configurations
-          # Each configuration includes all parameters needed by the build job
-          full_matrix = [
-              {
-                  "compiler_id": "gcc-11-libstdcxx",
-                  "compiler": "gcc",
-                  "cc": "gcc-11",
-                  "cxx": "g++-11",
-                  "compiler_version": 11,
-                  "stdlib": "default",
-                  "configuration": "Debug",
-                  "job_type": "build"
-              },
+          matrix = [
               {
                   "compiler_id": "gcc-13-libstdcxx",
                   "compiler": "gcc",
@@ -93,17 +64,6 @@ jobs:
                       "grpc/*": ["-Wno-missing-template-arg-list-after-template-kw"]
                   }
               },
-              {
-                  "compiler_id": "clang-14-libstdcxx-gcc11",
-                  "compiler": "clang",
-                  "cc": "clang-14",
-                  "cxx": "clang++-14",
-                  "compiler_version": 14,
-                  "stdlib": "libstdcxx",
-                  "clang_gcc_toolchain": 11,
-                  "configuration": "Debug",
-                  "job_type": "build"
-              },
               {
                   "compiler_id": "clang-16-libstdcxx-gcc13",
                   "compiler": "clang",
@@ -114,117 +74,8 @@ jobs:
                   "clang_gcc_toolchain": 13,
                   "configuration": "Debug",
                   "job_type": "build"
-              },
-              {
-                  "compiler_id": "clang-17-libcxx",
-                  "compiler": "clang",
-                  "cc": "clang-17",
-                  "cxx": "clang++-17",
-                  "compiler_version": 17,
-                  "stdlib": "libcxx",
-                  "configuration": "Debug",
-                  "job_type": "build"
-              },
-              {
-                  # Clang 18 - testing if it's faster than Clang 17 with libc++
-                  # Requires patching Conan v1 settings.yml to add version 18
-                  "compiler_id": "clang-18-libcxx",
-                  "compiler": "clang",
-                  "cc": "clang-18",
-                  "cxx": "clang++-18",
-                  "compiler_version": 18,
-                  "stdlib": "libcxx",
-                  "configuration": "Debug",
-                  "job_type": "build"
               }
           ]
-          
-          # Minimal matrix for PRs and feature branches
-          minimal_matrix = [
-              full_matrix[1],  # gcc-13 (middle-ground gcc)
-              full_matrix[2],  # clang-20 llvm-cov coverage
-              full_matrix[3]   # clang-14 (mature, stable clang)
-          ]
-          
-          # Determine which matrix to use based on the target branch
-          ref = "${{ github.ref }}"
-          base_ref = "${{ github.base_ref }}"  # For PRs, this is the target branch
-          event_name = "${{ github.event_name }}"
-          pr_title = """${{ steps.escape.outputs.title }}"""
-          pr_labels = """${{ join(github.event.pull_request.labels.*.name, ',') }}"""
-          pr_head_sha = "${{ github.event.pull_request.head.sha }}"
-
-          # Get commit message - for PRs, fetch via API since head_commit.message is empty
-          if event_name == "pull_request" and pr_head_sha:
-              try:
-                  url = f"https://api.github.com/repos/${{ github.repository }}/commits/{pr_head_sha}"
-                  req = urllib.request.Request(url, headers={
-                      "Accept": "application/vnd.github.v3+json",
-                      "Authorization": f"Bearer {os.environ.get('GH_TOKEN', '')}"
-                  })
-                  with urllib.request.urlopen(req) as response:
-                      data = json.load(response)
-                      commit_message = data["commit"]["message"]
-              except Exception as e:
-                  print(f"Failed to fetch commit message: {e}")
-                  commit_message = ""
-          else:
-              commit_message = """${{ github.event.head_commit.message }}"""
-
-          # Debug logging
-          print(f"Event: {event_name}")
-          print(f"Ref: {ref}")
-          print(f"Base ref: {base_ref}")
-          print(f"PR head SHA: {pr_head_sha}")
-          print(f"PR title: {pr_title}")
-          print(f"PR labels: {pr_labels}")
-          print(f"Commit message: {commit_message}")
-          
-          # Manual trigger input to force full matrix.
-          manual_full = "${{ github.event.inputs.full_matrix || 'false' }}" == "true"
-
-          # Label/manual overrides, while preserving existing title/commit behavior.
-          force_full = (
-              manual_full
-              or "[ci-nix-full-matrix]" in commit_message
-              or "[ci-nix-full-matrix]" in pr_title
-              or ("ci-full-build" in pr_labels and "ci-nix-full-matrix" in pr_labels)
-          )
-          force_min = (
-              "ci-full-build" in pr_labels
-          )
-          print(f"Force full matrix: {force_full}")
-          print(f"Force min matrix: {force_min}")
-          
-          # Check if this is targeting a main branch
-          # For PRs: check base_ref (target branch)
-          # For pushes: check ref (current branch)
-          main_branches = ["refs/heads/dev", "refs/heads/release", "refs/heads/candidate"]
-          
-          if force_full:
-              # Override: always use full matrix if forced by manual input or label.
-              use_full = True
-          elif force_min:
-              # Override: always use minimal matrix if ci-full-build label is present.
-              use_full = False
-          elif event_name == "pull_request":
-              # For PRs, base_ref is just the branch name (e.g., "dev", not "refs/heads/dev")
-              # Check if the PR targets release or candidate (more critical branches)
-              use_full = base_ref in ["release", "candidate"]
-          else:
-              # For pushes, ref is the full reference (e.g., "refs/heads/dev")
-              use_full = ref in main_branches
-          
-          # Select the appropriate matrix
-          if use_full:
-              if force_full:
-                  print(f"Using FULL matrix (7 configs (build x6 + clang-20 llvm-cov coverage)) - forced by [ci-nix-full-matrix] tag")
-              else:
-                  print(f"Using FULL matrix (7 configs (build x6 + clang-20 llvm-cov coverage)) - targeting main branch")
-              matrix = full_matrix
-          else:
-              print(f"Using MINIMAL matrix (3 configs) - feature branch/PR")
-              matrix = minimal_matrix
 
           # Add runs_on based on job_type
           for entry in matrix: