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>

packages/contracts/src/message-metadata.ts

@@ -0,0 +1,45 @@
+// Single source of truth for cross-app message metadata contracts.
+// ErrorReason + MessageMetadata: sentinel shapes stored in messages.metadata
+// and carried on WS frames. AgentSessionConfig: the required/nullable shape
+// used by CoderPane/AgentComposerBar for provider dispatch.
+
+export type ErrorReason =
+  | 'llm_provider_error'
+  | 'tool_execution_failed'
+  | 'summary_after_cap_failed';
+
+export type MessageMetadata =
+  | {
+      kind: 'cap_hit';
+      used: number;
+      limit: number;
+      agent_name: string | null;
+      can_continue: boolean;
+    }
+  | {
+      kind: 'doom_loop';
+      tool_name: string;
+      args: Record<string, unknown>;
+      threshold: number;
+    }
+  | {
+      kind: 'mistake_recovery';
+      failure_kinds: string[];
+      count: number;
+      escalated: boolean;
+      can_continue?: boolean;
+    }
+  | {
+      kind: 'error';
+      error_reason: ErrorReason;
+      error_text: string;
+    };
+
+// Unified definition is the web required/nullable shape (the coder's all-optional
+// copy was dead — zero importers in apps/coder/src).
+export interface AgentSessionConfig {
+  provider: string;
+  model: string;
+  modeId: string | null;
+  thinkingOptionId: string | null;
+}