docs: Drop redundant repo-level AGENTS.md pointers, shorten Build section

- The harness auto-loads every ancestor AGENTS.md (including root), so
  a nested file telling an agent to "see the repo-level AGENTS.md" is
  pure overhead — it can trigger a re-load/duplication-check/discard
  cycle, and some models just retain the duplicate content in context.
  Drop that pointer from every nested AGENTS.md; keep pointers to
  things that are NOT auto-loaded (README.md, docs/consensus.md,
  non-ancestor AGENTS.md like tx/ -> rpc/).
- Root AGENTS.md's Build section is now a short index pointer to
  BUILD.md and docs/build/nix.md instead of restating Nix setup steps,
  per review feedback that AGENTS.md should mostly index other docs
  rather than duplicate them.

Addresses PR review comments:
https://github.com/XRPLF/rippled/pull/8198#discussion_r3979325914
https://github.com/XRPLF/rippled/pull/8198#discussion_r3979424992

include/xrpl/consensus/AGENTS.md

@@ -1,3 +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.
+See [README.md](./README.md) for a short pointer, and [docs/consensus.md](../../../docs/consensus.md) for the full consensus design.

include/xrpl/nodestore/AGENTS.md

@@ -1,3 +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.
+See [README.md](./README.md) for backend and benchmark design.

include/xrpl/shamap/AGENTS.md

@@ -1,3 +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.
+See [README.md](./README.md) for the SHAMap design.