docs(engineer): document the .simard-engineer-claim two-layer exclusion (#2621)

[rysweet]

Summary

Docs-only follow-up to the merged fix for #2621 (delivered by #2623, commit 2c32fe65).

Adds docs/reference/engineer-claim-sentinel-exclusion.md, a reference page that documents the two-layer defense keeping Simard's private per-worktree liveness sentinel (.simard-engineer-claim) from ever registering as an uncommitted change — so the engineer-loop pre-mutation guard cannot trip on it in a target repo that does not gitignore the file.

What it documents

• The deadlock — how a single ?? .simard-engineer-claim untracked entry made worktree_dirty == true, aborting every mutating engineer with DirtyWorktree before the agent spawned, and driving the infinite re-dispatch loop.
• Layer 1 (creation-time) — exclude_engineer_claim() appends a root-anchored /.simard-engineer-claim to the worktree's info/exclude (resolved via git rev-parse --git-path, idempotent, lock-guarded, non-fatal on failure).
• Layer 2 (read-time) — strip_claim_sentinel() drops the exact-root sentinel from every git status consumer (inspect_workspace, verify_agent_spawn_artifacts) before worktree_dirty is computed.
• The ENGINEER_CLAIM_FILE single-source-of-truth constant, security properties, and the pinning regression tests in tests_claim_sentinel.rs / tests_extra.rs.

Scope

• Additive/surgical: one new reference page + one mkdocs.yml nav entry.
• No code change. No new println!/eprintln!. No "Bridge" names.
• mkdocs build --strict passes locally (no broken links / nav warnings).
• Every symbol, test name, and cross-link in the page was verified against the merged source.

Refs #2621.