feat: implement Hook Chains runtime integration for self-healing agent mesh MVP (#711)

* feat: implement Hook Chains runtime integration for self-healing agent mesh MVP

- Add Hook Chains config loader, evaluator, and dispatcher in src/utils/hookChains.ts
- Wire PostToolUseFailure hook dispatch in executePostToolUseFailureHooks()
- Wire TaskCompleted hook dispatch in executeTaskCompletedHooks()
- Integrate fallback-agent launcher with permission preservation (canUseTool threading)
- Add safety hardening for config-read errors (try-catch protection)
- Update docs with MVP runtime trigger explanation
- Add 10 unit tests and 4 integration tests covering config, rules, guards, and actions

This completes the self-healing agent mesh MVP by enabling declarative rule-based
responses to tool failures and task completions, with fallback agent spawning,
team notification, and capacity warming actions.

* Update docs/hook-chains.md

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* Update src/utils/hookChains.ts

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* fix: address PR #711 review blockers for Hook Chains

- Gate hook-chain dispatch behind feature('HOOK_CHAINS') and default env gate to off
- Remove committed local artifact (agent.log) and ignore it in .gitignore
- Revert hook dispatcher signature threading changes for canUseTool
- Use ToolUseContext metadata hookChainsCanUseTool for fallback launch permissions
- Make spawn_fallback_agent fail explicitly when launcher context is unavailable
- Add config cache max age and guard map size limits to bound runtime memory
- Update docs and tests for default-off gating and explicit fallback failure

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

src/Tool.ts

@@ -249,6 +249,11 @@ export type ToolUseContext = {
   /** When true, canUseTool must always be called even when hooks auto-approve.
    *  Used by speculation for overlay file path rewriting. */
   requireCanUseTool?: boolean
+  /**
+   * Optional callback used by hook-chain fallback actions that launch
+   * AgentTool from hook runtime paths.
+   */
+  hookChainsCanUseTool?: CanUseToolFn
   messages: Message[]
   fileReadingLimits?: {
     maxTokens?: number