fix(context): validate each find_teamspaces candidate's ticket custom type

[antonisobkowicz-asana]

Summary

Fixes CODE-1224: find_teamspaces returned candidates from Asana's typeahead endpoint with a blanket schema_validated: false on every response, regardless of whether a given candidate was actually configured as a Command Teamspace. An agent could select a plain project and only discover it was wrong on the next call, hitting schema_incompatible: "No ticket custom type found" — which is literally what happened to me twice in an earlier session on this same repo.

• src/schema_discovery.ts: new exported hasResolvableTicketCustomType(), reusing the existing collectCustomTypes/resolveTicketCustomType logic (no duplicated logic) — one Asana call per candidate (customTypes.getCustomTypesWithHttpInfo({project: gid})), much cheaper than full schema discovery (skips custom-field-settings pagination and the per-Release project reads). Any CommandError from the check (incompatible, ambiguous, or otherwise) is treated as "not usable" rather than aborting the whole find_teamspaces call.
• src/tools/context.ts: moved schema_validated from a top-level z.literal(false) onto each candidate as a real boolean, computed sequentially (one call per candidate, bounded by the existing deadline rather than bursting up to 20 concurrent requests) after the typeahead call.
• src/tool_definitions/context.ts / README.md: updated the tool's advertised description to reflect the new, meaningful per-candidate signal instead of the old "not schema-validated" disclaimer.
• Tests: new unit tests for hasResolvableTicketCustomType (true/false/ambiguous) in tests/schema_discovery.test.ts, a new tests/context.test.ts test proving mixed valid/invalid candidates are correctly distinguished (and that each gets its own customTypes lookup with the right project GID), plus updated existing tests/contract tests for the new output shape.

Breaking change, per AGENTS.md's "preserve the public MCP contract unless a task explicitly authorizes a breaking change" — find_teamspaces's output shape changes: the top-level schema_validated field is gone, replaced by a per-candidate one. Investigated non-validating and filter-based alternatives before choosing this approach; see the ticket/PR discussion for the tradeoffs (no native multi-project batching in the typed Asana SDK, so worst case is up to 20 sequential extra calls, bounded by the existing per-request deadline).

Test plan

• npx tsc --noEmit
• npm run lint
• npm test — 387 passing
• npm run build
• npm run check (full gate)

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