Add a gated Known Limitations and Deferred Work section to every package README

Every packages/*/* README now carries a canonical '## Known Limitations and
Deferred Work' section: condensed, evidence-backed bullets for consumer-visible
gaps (unimplemented features, platform caveats, MVP cuts) and consciously
postponed work (TODO/FIXME/XXX markers, RFC deferrals still open). The ten
pre-existing ad-hoc variants ('What is NOT here (TODO)', 'Deferred',
'Limitations (MVP)', 'Known limitations (tracked TODOs)', ...) are normalized
into the canonical heading.

A new doc-sync gate, scripts/verify-readme-limitations.ts, enforces the shape:
exactly one limitations-like heading per package README, byte-equal to the
canonical h2, with at least one bullet; near-miss headings fail so variants
cannot creep back. Packages with genuinely nothing to declare (dsh-brand,
dsh-timeout, dsh-subagent-mock, dsh-app-boot) are whitelisted in the script and
must NOT carry the section; whitelist entries are validated against the scanned
package set so a rename fails loud.

Wired into the doc-sync chain (package.json) and the run-gates doc-sync leaf
set; the standing rule lands in packages/AGENTS.md and the adding-a-package
cookbook; decision record in
docs/rfc/implemented/process/2026-07-10-readme-known-limitations-gate.md
(RFC index regenerated).

Also fixes two stale '(deferred)' markers claiming dsh-compact-basic is
unimplemented (the dsh-compact seam README's package table and the seam's
module doc comment).

packages/session-persistence/session-persistence/README.md

@@ -49,3 +49,9 @@ Three backends run these suites: an in-memory reference (in `tests/`), `dsh-sess
 ## Metadata types
 
 Re-exported from `dsh-session`: `SessionHeader` (immutable session metadata: `version`, `id`, `createdAt`, `cwd?`, `parentSession?`, `seedLength?`).
+
+## Known Limitations and Deferred Work
+
+- **No deletion or retention surface** — the seam is `create`/`append`/`load`/`list` only; pruning stored sessions is out-of-band backend maintenance.
+- **`list()` is unpaginated and unfiltered** — it returns every stored session's header; fine for local stores, unindexed at scale.
+- **Repair-time synthetic closers are the only crash story** — a backend must synthesize `tool/result`/`step/end`/`turn/end` closers on load; there is no partial-turn resume that continues an interrupted turn instead of closing it.