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/subagent/subagent/README.md

@@ -36,8 +36,9 @@ Beside `capabilities` sits one DESCRIPTIVE fact, not validated by the service: `
 
 The service also announces provider lifecycle: `subagent/provider-added` (the frozen registry snapshot) fires after a registration and `subagent/provider-removed` (the accepted name) after an unregistration, so a consumer deriving state from a named provider (the model-facing tool wording) mirrors registry membership instead of assuming load order — the cordis Loader starts sibling plugins concurrently, so "listed earlier" does not mean "registered earlier". Run lifecycle is gated by provider readiness: `subagent/start` (payload `SubagentRunInfo`) fires only after `run.started` fulfills, and `subagent/end` (payload `SubagentRunEndInfo`) fires only for that announced run; readiness rejection emits neither. For spawn/fork, the start listener can resolve the published child via `ctx.agents.get(info.id)`; a remote provider need not have a local registry entry. Both events are **observe-only** plain emits. The service observes `result` immediately even while readiness is pending, clones its output before the caller can mutate it, and buffers that end payload until start has fired; a rejecting result cannot become an unhandled detached promise, start always precedes end, and a listener cannot corrupt the caller's result. `subagent/end` carries the cloned output as `lastAssistantMessage` on the settle path and omits it on infrastructure rejection. Any run-affecting decision is out of scope for this observe-only surface.
 
-## Scope (first cut)
+## Known Limitations and Deferred Work
 
-The consumer collects **synchronously**: it starts a run and awaits `result`. Steering (`sendMessage`) is part of the contract but intentionally unused. Background / poll / spill semantics are deferred to a future redesign unifying long-running-tool handling across subagents and bash. See the RFC: [docs/rfc/implemented/feature/2026-06-21-subagent-capability-seam.md](../../../docs/rfc/implemented/feature/2026-06-21-subagent-capability-seam.md).
+- **The consumer collects synchronously** — it starts a run and awaits `result`; steering (`sendMessage`) is part of the contract but intentionally unused, and background / poll / spill semantics are deferred to a future redesign unifying long-running-tool handling across subagents and bash ([the seam RFC](../../../docs/rfc/implemented/feature/2026-06-21-subagent-capability-seam.md)).
+- **The lifecycle events are observe-only** — a run-affecting `subagent/end` (an awaited continuation/decision surface) is deferred until a consumer needs one (`FIXME(subagent-continuation)`).
 
 See `src/types.ts` for the full contracts.