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>

package.json

@@ -5,7 +5,7 @@
   "scripts": {
     "dev:server": "pnpm --filter ./apps/server dev",
     "dev:web": "pnpm --filter ./apps/web dev",
-    "build": "pnpm --filter ./apps/web build && pnpm --filter ./apps/server build",
+    "build": "pnpm --filter ./packages/contracts build && pnpm --filter ./apps/web build && pnpm --filter ./apps/server build",
     "start": "node apps/server/dist/index.js"
   },
   "devDependencies": {