docs: remove agent-generated planning docs and forbid committing them (#4771)

docs: remove agent-generated planning docs, forbid committing them

Delete one-shot planning/progress artifacts that were checked into the tree:
the 14 superpowers plan/tracker docs under docs/superpowers/plans/, plus
issue-scoped implementation plans, optimization conclusions, and dated
benchmark-result snapshots under docs/ (issue-4003 ListObjectsV2 plans,
get-small-file conclusion, issue824/issue829 benchmark results, issue-713
>1GiB GET baseline summary and ops guide).

Codify the rule so they do not come back:
- .gitignore drops the docs/superpowers whitelist, so anything new under
  docs/ stays ignored unless force-added.
- AGENTS.md gains an explicit 'do not commit planning-type documents' rule
  scoping version control to the durable architecture/operations/testing sets.
- docs/architecture/README.md, overview.md, arch-checks SKILL.md, and
  check_doc_paths.sh drop their references to the removed archive.

docs/architecture/README.md

@@ -6,8 +6,9 @@ contracts, boundary rules, and support matrices.
 Two rules keep this directory healthy:
 
 1. **Durable reference only.** One-shot implementation plans, task trackers,
-   and PR templates live in [`docs/superpowers/plans/`](../superpowers/plans/)
-   and are archived there when their work closes.
+   and PR templates do not belong in the repository — keep them in the issue
+   tracker or your local worktree. When their work closes, delete them rather
+   than archiving them here.
 2. **No copies of other sources of truth.** Crate lists come from
    `Cargo.toml`, CI steps from `.github/workflows/ci.yml`, code structure from
    the code. `scripts/check_doc_paths.sh` fails the pre-commit gate when a
@@ -54,9 +55,8 @@ Two rules keep this directory healthy:
 - [compat-cleanup-register.md](compat-cleanup-register.md)
 
 Historical plans and trackers (rebalance/decommission phases,
-migration-progress ledger) were moved to
-[`docs/superpowers/plans/`](../superpowers/plans/) in 2026-07. One-shot
-migration snapshots whose only consumer was that ledger (startup timeline,
-scheduler baseline, profiling/NUMA capability inventory, KMS development
-defaults inventory) followed once the architecture-review ledger they fed
-closed out (backlog#660/#665).
+migration-progress ledger, and the one-shot migration snapshots that fed it —
+startup timeline, scheduler baseline, profiling/NUMA capability inventory, KMS
+development defaults inventory) were retired in 2026-07 once the
+architecture-review ledger they served closed out (backlog#660/#665). Planning
+documents are no longer kept in the repository.

docs/architecture/overview.md

@@ -50,8 +50,6 @@ hot-path behavior must not drift during this migration.
 - [`config-model-boundary-adr.md`](config-model-boundary-adr.md): target crate,
   module path, dependency rules, and verification gates for moving the pure
   server-config model.
-- [`migration-progress.md`](../superpowers/plans/migration-progress.md): historical
-  task ledger for the closed migration (execution tracking moved to issue #665).
 - [`compat-cleanup-register.md`](compat-cleanup-register.md): temporary
   compatibility code that must be removed later.