fix(sdk): repair tool schemas via cross-tool $defs pool with transitive closure

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Problem

The hosted Stitch MCP server (https://stitch.googleapis.com/mcp) emits a tools/list in which upload_design_md's outputSchema references #/$defs/ScreenInstance but ships no $defs block (see #367). Clients that eagerly compile tool schemas — AJV inside @modelcontextprotocol/sdk, and strict clients such as kimi-code — fail schema registration and drop all 15 tools, making the integration unusable.

The SDK's existing repair (WELL_KNOWN_DEFS stubs, #353/#355) prevents the crash, but only by injecting hardcoded partial shapes that have drifted from the backend:

• SelectedScreenInstance stub has screenId/instanceId; the backend's real shape is id/sourceScreen with required: ["id", "sourceScreen"].
• ScreenInstance stub is missing isResized, needsLayout, textContent, the recursive variantScreenInstance property, and the TEXT_INSTANCE enum member.
• File stub omits the backend's userFeedback → #/$defs/UserFeedback ref — one-shot stub injection cannot represent defs that introduce new refs.
• Any future dangling def not on the hardcoded list crashes again.

Fix

Repair dynamically from the response itself, stubs only as fallback:

1. collectDefPool(tools) (new, exported): harvests every $defs entry across all input/output schemas in the tools/list response. The backend defines each entity properly in at least one tool's schema (e.g. create_project's outputSchema has the real ScreenInstance), so the pool captures the authoritative shapes.
2. repairSchema(schema, defPool?): resolves missing defs from the pool first (real backend definitions, deep-cloned so schemas share no mutable state), falling back to WELL_KNOWN_DEFS. Iterates to a fixpoint so injected defs that introduce new refs (File → UserFeedback) are fully repaired. Signature is backward compatible.
3. Fallback stubs synced with the live backend shapes (captured from tools/list on 2026-08-21) so the fallback path is faithful too.

Verification

• 21 schema-repair unit tests (11 new): pool harvest, pool-beats-stub, transitive closure, stub fallback, no-overwrite, deep-copy isolation, and a regression test modeling the exact upload_design_md shape from MCP tools/list emits unresolvable $ref in outputSchema (upload_design_md dangling, ScreenInstance recursive) — clients drop all tools #367.
• Full package suite: 198/198 passing; tsc --noEmit clean; prettier clean.
• Real-world check: applied repairToolSchemas to a live tools/list captured from the hosted endpoint today — all 15 tools' schemas fully resolve after repair (previously upload_design_md's outputSchema was unresolvable). This also unblocks strict non-SDK clients (kimi-code) when the SDK/proxy layer sits in front.

Mitigates #367 — the hosted server still needs the server-side schema fix, but this makes every SDK and proxy consumer immune to it.

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