feat(contracts): introduce proto contracts tree and wire.v1 envelope

[keugenek]

Summary

Adds contracts/ at the repo root as the source of truth for cross-component
interface shapes described in the design doc.
This PR ships the first slice (appkit/v1/wire.proto) and the build wiring
needed for subsequent contracts to land without further monorepo changes.

What's included

• contracts/buf.yaml + contracts/buf.gen.yaml — buf module config and codegen
  pipeline using @bufbuild/protoc-gen-es (local, no remote plugin resolution).
• contracts/appkit/v1/wire.proto — SseEnvelope, SseError, ResultFormat enum.
  Covers the SSE envelope, structured error frames, and SQL warehouse
  result-format negotiation. Route conventions, heartbeat/reconnect semantics,
  and the window.__appkit__ client-config payload are deferred to follow-ups.
• packages/contracts/ — new workspace package @databricks/appkit-contracts.
  Runs buf generate via tools/generate-contracts.ts and re-exports the
  typed bindings. Built with tsdown; follows existing packages/shared/
  conventions.
• packages/appkit/src/stream/types.ts — imports the generated SseEnvelope
  and adds a compile-time compatibility anchor against the existing
  BufferedEvent. Non-breaking; BufferedEvent will retire in favour of the
  generated type in a follow-up.
• Root package.json — contracts:generate and contracts:lint scripts.
• docs/docs/contracts.md — short intro page linking back to the design doc
  and outlining the four planned follow-up contracts.

Interface design

Per the design doc, the contracts tree will cover five interfaces:

Contract	Status	Replaces
appkit/v1/wire.proto	this PR	hand-maintained SSE types in packages/appkit/src/stream/types.ts
appkit/v1/query.proto	follow-up	TS-only QueryRegistry augmentation from the type-generator
appkit/v1/plugin.proto	follow-up	packages/shared/src/schemas/plugin-manifest.schema.json
agenteval/v1/*.proto	follow-up	ad-hoc JSON between eval framework and apps
appkit/v1/deploy.proto	follow-up	implicit app.yaml contract with `databricks apps deploy`

Consumers: the TS backend, future Python/Rust ports, the React SDK, the
`databricks` CLI, Claude Code scaffolding skills, and the agent-eval framework
all compile against the same generated bindings.

Verification

• `pnpm contracts:lint` — buf lint clean.
• `pnpm contracts:generate` — produces `packages/contracts/src/generated/appkit/v1/wire_pb.ts`.
• `pnpm -r typecheck` — all 8 workspace projects pass.
• `pnpm build` — full monorepo build, `@databricks/appkit-contracts` emits dist/, publint clean.
• `pnpm test` — 1564 tests pass (no new or changed tests in this PR).
• `pnpm knip` + `pnpm check` — clean.

Test plan

• Regenerate locally: `pnpm contracts:generate` and verify `packages/contracts/src/generated/appkit/v1/wire_pb.ts` is produced deterministically.
• Intentionally break the contract (e.g. rename `SseEnvelope.id` to `eventId` in the proto, regenerate) and confirm `pnpm -r typecheck` fails on the `_BufferedEventSseEnvelopeCompat` anchor.
• Confirm `pnpm build` produces `packages/contracts/dist/` with `.d.ts` + `.js` for the generated types.

Follow-ups

See design doc for the full five-contract plan. Next PRs:

1. `appkit/v1/query.proto` — retires the TS-only `QueryRegistry` augmentation.
2. `appkit/v1/plugin.proto` — retires `plugin-manifest.schema.json`.
3. `agenteval/v1/*.proto` — lands alongside the eval-framework work.
4. `appkit/v1/deploy.proto` — coordinates with the `databricks` CLI team.

This pull request and its description are AI-assisted by Isaac.