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/web/web/README.md

@@ -42,3 +42,10 @@ The failure branches throw `WebError`, whose structured code (plus message detai
 ## Vocabulary
 
 `WebSearchRequest` (`query`, `maxResults?`) → `WebSearchResult` (`providerId`, `query`, `content?`, `sources[]`, `truncated`); each `WebSearchSource` has a required `url` and optional `title`/`snippet`/`publishedAt` (Perplexity citations may be URL-only). `WebFetchRequest` (`url`, `timeoutMs?`) → `WebFetchResult` (`providerId`, final `url`, `statusCode`, `body`, `truncated`); `WebFetchBody` is a CLOSED discriminated union (`html` | `text`) owned here — consumers `switch` to exhaustiveness so a new kind breaks their compilation until handled. See `src/types.ts` for the full contracts and the `WebError` code taxonomy.
+
+## Known Limitations and Deferred Work
+
+- **No observation surface** — no provider-change event and no capability-status query; availability is observed only by executing `search()`/`fetch()` and routing the thrown `WebError` codes, and the no-provider failure is the generic `WEB_PROVIDER_UNAVAILABLE` with no per-provider reason enumeration ([RFC](../../../docs/rfc/implemented/simplification/2026-07-04-drop-unconsumed-web-observation-surface.md)).
+- **`WebSearchRequest` carries only `query` + `maxResults`** — provider-neutral controls (recency, domain filters, regional hints, search depth) are deferred until Exa and Perplexity can both honor them honestly ([seam RFC](../../../docs/rfc/implemented/architecture/2026-06-24-web-capability-seam.md)).
+- **`WebFetchBody` has no `pdf` arm** — text-extractable PDF support is named deferred work; the closed union makes adding it a compile-enforced change across the three web packages.
+- **Provider-backed page extraction is out of scope of `fetch()`** — a Firecrawl/Tavily-style `web_extract` capability is deferred rather than widening the fetch seam.