chore(centaur-client): give the package a typecheck script so pnpm check covers it

[gbasin]

centaur-client was the only workspace package with no typecheck script. pnpm -r typecheck skips a package that lacks the script rather than failing, so pnpm check — the pre-PR bar CLAUDE.md tells you to run — has never typechecked this package at all. Its types were only ever checked by build (bare tsc -p tsconfig.json, whose include covers src/**/*.ts and therefore *.test.ts), which runs only in CI's Surface static job.

That is exactly how #545 shipped a red build: vitest doesn't typecheck, so a type error in a test file was invisible to every gate a human runs locally and only surfaced in CI.

src/toolDisplay.test.ts(5,82): error TS2741: Property 'executionId' is missing in type
'{ type: "tool_call"; ... }' but required in type 'ToolCallItem'.

Verification

Re-introduced that exact error on this branch and ran the gate that previously missed it:

• with the bug → pnpm -r typecheck prints centaur-client typecheck: Failed + the TS2741, exits 2
• without it → exits 0
• pnpm --filter @atrium/centaur-client typecheck leaves no dist/; build still emits it (--noEmit because build owns outDir + declaration — a typecheck shouldn't race it for artifacts)
• pnpm lint clean

Note

build and typecheck now both compile the same project, so the work is duplicated in CI. That's deliberate: build exists to produce dist for publishing, typecheck exists to be part of pnpm check. Collapsing them would mean either pnpm check writes build artifacts or Surface static stops producing them.