docs: Add subsystem AGENTS.md files for transactors, ledger helpers, and READMEs

Round 2 of PR-review mining: found two subsystem-specific, unrecorded
conventions (pseudo-account auth/freeze exemption in vault/lending
transactors; SLE-type assertion in ledger helpers) that warrant their
own nested AGENTS.md rather than more root-level bullets.

Also added thin AGENTS.md pointer files (with CLAUDE.md symlinks,
matching the existing convention) for subsystems that already have a
substantial README.md but no AGENTS.md — agent tooling auto-loads
AGENTS.md when working in a directory but not README.md, so those
conventions were otherwise invisible to an agent that didn't think to
look.

include/xrpl/consensus/AGENTS.md

@@ -0,0 +1,3 @@
+# AGENTS.md — consensus
+
+See the repo-level [AGENTS.md](../../../AGENTS.md) for general guidance. See [README.md](./README.md) for a short pointer, and [docs/consensus.md](../../../docs/consensus.md) for the full consensus design.

include/xrpl/consensus/CLAUDE.md

@@ -0,0 +1 @@
+AGENTS.md

include/xrpl/nodestore/AGENTS.md

@@ -0,0 +1,3 @@
+# AGENTS.md — nodestore
+
+See the repo-level [AGENTS.md](../../../AGENTS.md) for general guidance and [README.md](./README.md) for backend and benchmark design.

include/xrpl/nodestore/CLAUDE.md

@@ -0,0 +1 @@
+AGENTS.md

include/xrpl/shamap/AGENTS.md

@@ -0,0 +1,3 @@
+# AGENTS.md — shamap
+
+See the repo-level [AGENTS.md](../../../AGENTS.md) for general guidance and [README.md](./README.md) for the SHAMap design.

include/xrpl/shamap/CLAUDE.md

@@ -0,0 +1 @@
+AGENTS.md