chore(docs): refresh agent docs, guard doc paths, archive plans (#4203)

Agent-instruction and architecture docs had drifted from the code:

- CLAUDE.md: slim to commands + pointers; fix wrong claim that
  `make pre-commit` is the full pre-PR gate (that is `make pre-pr`);
  drop stale pre-#3929 file paths and merged bug narratives
- AGENTS.md: drop dead `rust-refactor-helper` skill rule and the
  hand-maintained (already stale) scoped-AGENTS index; link
  architecture docs from Sources of Truth
- .github/AGENTS.md: replace the outdated copied CI command matrix
  with a pointer to ci.yml
- crates/AGENTS.md: merge duplicated Testing sections
- ARCHITECTURE.md: resolve the utils->config contradiction (edges are
  removed), mark volatile counts as snapshots, fix a bad path
- docs/architecture: add README router; move one-shot plans/trackers
  (rebalance-decommission phases, migration-progress ledger, PR
  template) to docs/superpowers/plans with archive headers; fix stale
  source paths in kept inventories (core/sets.rs, core/pools.rs,
  store/mod.rs, startup_* split from #3671)
- docs/operations/tier-ilm-debugging.md: extracted tier debugging
  playbook with corrected paths
- scripts/check_doc_paths.sh: new guard failing pre-commit/pre-pr when
  instruction/architecture docs reference nonexistent file paths
- .claude/skills: add tier-debug and arch-checks repo skills;
  .gitignore now keeps .claude/skills and docs/operations committable

Verification: ./scripts/check_doc_paths.sh,
./scripts/check_architecture_migration_rules.sh (both pass)

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

crates/AGENTS.md

@@ -8,12 +8,6 @@ Applies to all paths under `crates/`.
 - Prefer `thiserror` for library-facing error types.
 - Do not use `unwrap()`, `expect()`, or panic-driven control flow outside tests.
 
-## Testing
-
-- Keep unit tests close to the module they test.
-- Keep integration tests under each crate's `tests/` directory.
-- Add regression tests for bug fixes and behavior changes.
-
 ## Error Type Design
 
 - Public API functions must return a typed error enum (preferably `thiserror`-derived), never `Result<_, String>`.