fix(core): concurrency cancellation, TOCTOU in callTool, defensive hook copies, and new tests - #6

…ok copies, and new tests

- concurrency.ts: add AbortSignal support and stop sibling workers on error
- tool-execution.ts: accept AbortSignal for early cancellation on stream abort
- prompt-request.ts: create AbortController in stream(), abort in finally; add
  .catch(() => {}) to detached promise chain; defensively copy newMessages at
  all hook call sites to prevent shared-mutable-state corruption
- agent.ts: fix TOCTOU race in callTool() — single get() lookup + callWithTool()
  instead of separate contains() + call() lookups
- tool-set.ts: add callWithTool() for atomic tool invocation with pre-resolved ref
- tool-execution.ts, mcp/tool.ts: Symbol.for() -> Symbol() to avoid cross-realm
  symbol collisions
- test: concurrency.test.ts (10 tests), compact.test.ts (12 tests),
  tool-set.test.ts (+4 TOCTOU tests), prompt-request.test.ts (+2 tests for
  unhandled rejection and message integrity)