fix: stop committing volatile graph stats (AGENTS/CLAUDE/arch-doc churn)

[ReidenXerx]

Problem

Teammates who pull see committed agent docs perpetually "modified" — even with no real change. Root cause: three committed files embed live graph stats that regenerate on every re-index, and each machine's graph differs slightly:

• docs/ARCHITECTURE.gitnexus.md — kit-generated (generate-arch-doc, runs on refresh)
• the <!-- gitnexus:start -->…<!-- gitnexus:end --> block the GitNexus analyze tool injects into AGENTS.md / CLAUDE.md (symbol / relationship / per-area counts)

Example diff a teammate hit after a routine refresh:

- indexed as enter-control (49749 symbols, 117601 relationships, 231 flows)
+ indexed as enter-control (50592 symbols, 119358 relationships, 232 flows)
- Components area (1103 symbols) ... Hooks (358)
+ Components area (1115 symbols) ... Hooks (368)

Fix

• Arch doc: gitignore docs/ARCHITECTURE.gitnexus.md (still regenerated locally, just not committed).
• AGENTS.md / CLAUDE.md: new .gnkit/lib/stabilize-agent-docs.mjs strips the volatile gitnexus:start block while preserving the user's own content and the kit's stable gitnexus-agent-kit contract block (can't gitignore these files — they hold real user instructions).
• Runs where analyze re-injects the block: sync-teaching (install + autonomous agent-refresh) and the pre-commit hook (after full-pdg), so both the working tree and commits stay clean.

Verified

• Unit tests: strip removes block + stats, keeps user + kit content, idempotent; gitignore includes the arch doc. 56/56 pass.
• Real-world: a 626-line CLAUDE.md (325 lines of user rules) → block + stats gone, user content and kit block intact.

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