docs(api): record the os:check decision for client-sdk.mdx, and fence its JSX as tsx

[yinlianghui]

Part of #11942

Why this is not "13 markers"

The card's scope was: opt the page's fences into check:skill-examples, compile, and repair
each red honestly, on the premise that "each red is a real doc-vs-SDK divergence, not a marker
problem."

Measured, that premise does not hold: not one of the 13 reds is a doc-vs-SDK divergence.
All 13 fences were marked, the gate run, and the tree reverted. Result: 128 diagnostics, zero
divergences, in two structural classes neither of which this page can fix.

Class 1 — the docs surface cannot resolve the SDK (3 fences)

check:skill-examples compiles per surface. content/docs/** belongs to the
skills + docs (@objectstack/spec) surface, whose resolutionDir and paths map derive from
@objectstack/spec alone — and @objectstack/spec does not depend on @objectstack/client
(confirmed: neither packages/spec/node_modules/@objectstack/ nor the root
node_modules/@objectstack/ exists).

Marking the single most self-contained fence on the page — a pure createFilter() builder chain
that is unambiguously correct SDK code — yields:

content/docs/api/client-sdk.mdx:485:30
error TS2307: Cannot find module '@objectstack/client' or its corresponding type declarations.

That is a surface-resolution gap, not drift. The only real fix is adding @objectstack/client to
that surface's selfPackages, i.e. editing the gate — explicitly out of scope for this card.

This is not a new observation: content/docs/kernel/runtime-services/data-service.mdx already
records the identical constraint in prose, for the identical reason, and leaves its own SDK block
deliberately unmarked.

Class 2 — deliberate continuation fragments (10 fences)

Quick Start establishes const client = new ObjectStackClient(...) once; every later fence
continues that implied context. Marking them reds with 93 x TS2304 Cannot find name 'client',
plus TS18004 (conversationId, messages, userId, orderId, recordId), TS18046 (error is
unknown in a catch), TS2591 (process) and TS7006.

Making these compile means hand-declaring the SDK's own types or injecting casts into prose whose
subject is the real API — which pins each example to itself and teaches worse code than the page
teaches now. The card forbids editing a correct example into something that merely compiles.

What this PR does deliver

1. It records the decision. The issue's actual claim is that "nobody has decided, page by
page, which side of that line each block is on." This PR decides it for this page and writes the
measurement down, as a contributor-facing MDX comment (renders to nothing), following the
data-service.mdx precedent. The next agent reads the answer instead of re-deriving it.

2. It defuses a measured landmine. The React Hooks block is JSX but was fenced as typescript.
The gate writes each block out with its fence's own extension, so a marker on it produced only
TS1xxx syntax errors — and tsc stops at syntax errors and never runs the semantic pass. Measured
consequence, on this page:

fences marked	React Hooks fence	syntax errors	semantic diagnostics
12 (JSX one excluded)	typescript	0	118
13 (all)	typescript	14	0
13 (all)	tsx	0	128

One mis-tagged fence suppresses type-checking for every marked block across skills/ and
content/docs/ — 227 blocks — not just this page. Retagging it tsx is correct on its own terms
(the block is JSX) and converts a whole-surface blackout into honest per-block diagnostics.
check:doc-authoring's FENCE_OPEN accepts tsx, so the block stays visible to it.

The marker becomes available here the day the docs surface can resolve @objectstack/client.

Verification

Baseline and final are identical and unchanged, as intended — this PR adds no markers:

256 marked example(s) across 99 file(s), 3 surface(s)
256 prose examples type-check across 3 surface(s)

All 20 gate families derived at the final commit c84df2983 by
node scripts/pm/dispatch-gates.mjs --repo objectstack-ai/objectstack (no hand-written path list),
plus check:nul-bytes and check:skill-examples — 22/22 green, each exit code captured before
any pipe. MDX parse confirmed by compiling the page with the workspace's own @mdx-js/mdx@3.1.1.

Docs-only; no published package changes, so skip-changeset rather than a changeset.

Every mutation run above was performed under an EXIT INT TERM restore trap, with the marker count
on disk asserted before each run and the tree confirmed pristine after.

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