fix(build): add the root tsconfig.json typecheck has always pointed at

[unfoundbox]

Stacked on #8 — that is the base branch, so this diff is only the config.
Merge #8 first.

The bug

npm run typecheck is tsc --noEmit -p tsconfig.json, and no root
tsconfig.json has ever been committed. It is not gitignored; it was simply
never added. The script has failed with TS5058 for everyone who has run it,
on every commit in the repo's history. CI never ran it, so nothing noticed.

The gap is not cosmetic. core and cdp are type-checked as a side effect of
emitting declarations during their build. mcp is bundled by esbuild alone,
which strips types without reading them — packages/mcp/src/index.ts had never
been type-checked at all.

What changed

• tsconfig.json: one noEmit pass over all three packages' src.
• pretypecheck: npm run build, which is load-bearing — mcp imports its
  siblings by package name and those resolve to their built declarations.
  Without it a fresh clone gets three TS2307s instead of an answer.
• A typecheck CI job. It needs no Chrome, so it reports in under a minute
  rather than after a browser download, and the script cannot rot again.
• CONTRIBUTING notes the git-worktree trap: a worktree with no node_modules
  of its own resolves @motionvector/* up to the main checkout, so mcp gets
  checked against whatever is built over there. It cost a bad push during this
  work.

Verification, in a cold clone

npm ci && npm run typecheck from cold	exit 0
bare tsc, nothing built	3× TS2307 — why pretypecheck exists
deliberate type error in mcp	caught, TS2322

test/*.mjs stays outside the config deliberately. Type-checking the tests is
worth doing and is not a config line — they reach for fields the public types
do not carry, e.g. census.targetType. Separate change.

🤖 Generated with Claude Code

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