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>

BOOCODER.md

@@ -104,7 +104,7 @@ Either way, **adding to config does NOT install the binary.** Until the CLI is o
 ### Deploy + smoke
 
 Two deploy targets:
-- **Routes (host service):** `pnpm -C apps/server build && pnpm -C apps/coder build && sudo systemctl restart boocoder`
+- **Routes (host service):** `pnpm -C packages/contracts build && pnpm -C apps/server build && pnpm -C apps/coder build && sudo systemctl restart boocoder`
 - **Web UI (container):** `docker compose up --build -d boocode`
 
 Green gate (verified across phases 1–5): `pnpm -C apps/coder test` (134 passing) `&& pnpm -C apps/coder build`.