feat: single-source cross-app wire contracts in @boocode/contracts (v2.7.13)

Move all hand-synced cross-app wire contracts into one built workspace
package, @boocode/contracts, consumed by server/web/coder/coder-web via
workspace:* + a per-subpath exports map. The ws-frames and provider-config
Zod schemas are schema-first (z.infer); MessageMetadata, ErrorReason,
AgentSessionConfig, the provider snapshot types, and WorktreeRiskReport are
each single-sourced. Deletes the byte-identical copies and their parity
tests, fixes a live AgentSessionConfig drift (coder dead copy removed,
unified to the web required/nullable shape), removes the dead pending_change
WS arms in the fallback SPA, and inverts the build order (contracts builds
first) across root build, Dockerfile, and the coder deploy docs. Reverses
the shared-package decision declined in v2.5.12.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/project-discovery.md

@@ -100,7 +100,7 @@
 - cli: `pnpm -C apps/coder cli` (`tsx src/cli.ts`)
 - typecheck: `pnpm -C apps/coder typecheck`
 - test: `pnpm -C apps/coder test` (vitest run)
-- deploy: `pnpm -C apps/server build && pnpm -C apps/coder build && sudo systemctl restart boocoder`
+- deploy: `pnpm -C packages/contracts build && pnpm -C apps/server build && pnpm -C apps/coder build && sudo systemctl restart boocoder`
 - tsconfig: `apps/coder/tsconfig.json` (NodeNext, `declaration: false`)
 - test config: `apps/coder/vitest.config.ts` (vitest ^3.0.0, env=node, `globals: false`, `fileParallelism: false`)
 - test pattern: `src/**/__tests__/**/*.test.ts`