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- .github/AGENTS.md: replace the outdated copied CI command matrix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>

.agents/skills/tier-debug/SKILL.md

@@ -0,0 +1,34 @@
+---
+name: tier-debug
+description: Debug ILM tiering / lifecycle transition issues — NoSuchVersion on tier GET, restore failures, xl.meta inspection, remote-tier versionId tracing. Use when investigating tiered/transitioned objects, warm backends, or transition metadata.
+---
+
+# Tier / ILM Debugging
+
+Full playbook: [docs/operations/tier-ilm-debugging.md](../../../docs/operations/tier-ilm-debugging.md)
+— read it before changing tier code.
+
+Quick moves:
+
+```bash
+# Inspect transition metadata on disk (one xl.meta per erasure shard disk)
+cargo run -p rustfs-filemeta --example dump_fileinfo -- "/path/to/{bucket}/{object}/xl.meta"
+
+# Trace what versionId is sent to the remote tier
+RUST_LOG=rustfs_ecstore::bucket::lifecycle=debug ./target/debug/rustfs …
+```
+
+Interpretation:
+
+- `transition_ver_id: <none>` → correct for an unversioned tier bucket; no
+  `versionId` must be sent on tier GET/DELETE.
+- `transition_ver_id: 00000000-…` (nil) → corrupt legacy write-back; readers
+  must filter it out, never send it.
+- Empty-string `transitioned-versionID` metadata under both
+  `x-rustfs-internal-*` and `x-minio-internal-*` keys → object went to an
+  unversioned tier bucket.
+
+Code entry points: `crates/ecstore/src/bucket/lifecycle/bucket_lifecycle_ops.rs`
+(ILM actions), `crates/ecstore/src/services/tier/` (warm backends),
+`crates/filemeta/src/filemeta/version.rs` (metadata read/write + regression
+tests).