Why this is core

The abort story currently stops at the tool boundary. Ctrl+C aborts the LLM stream and the loop, but a running bash command (npm install, a test suite), a sub-agent, or a large directory walk ignores it — the agent waits for the tool to finish, so ^C appears broken exactly when users need it most. This PR makes cancellation flow from the agent, through every tool, down to the child process.

Changes

Tool.execute(args, signal?) — signal plumbing (src/tools/base.ts, src/agent.ts)

• The Tool interface now accepts an optional AbortSignal; execTool passes the agent's signal into every tool.
• execTool rethrows AbortError instead of stringifying it into a tool result. Previously a tool-level abort would have been fed back to the model as Error executing bash: AbortError and the turn would limp on.
• After tool results are pushed, an abort that fired during execution ends the turn immediately (no wasted LLM round); pending replies are backfilled with [interrupted] as before.

bash — actually kill the command (src/tools/bash.ts)

• Replaced exec with spawn so we own the process. shell: true makes the real command a grandchild of the shell; a plain child.kill() kills only the shell, and the command keeps running — and keeps holding the stdout pipe, so even exec's callback waits for it.
• POSIX: detached: true makes the shell a process-group leader; kill(-pid) kills the whole group.
• Windows: taskkill /pid <pid> /T /F walks the process tree (verified: a 10s node -e command now aborts in ~450ms instead of 10s).
• Kept the 16MB output cap (now explicit, with a [output truncated] marker), and the timeout now kills the tree too.

agent (sub-agent tool) — propagate the signal (src/tools/agent.ts)

• The parent's signal is passed into sub.chat(); an abort during the sub-agent's run propagates as AbortError (re-thrown, not stringified), so ^C cancels a runaway sub-agent too.

glob / grep — prompt walk cancellation

• Both walks (up to 20k/5k entries) check the signal per iteration and on entry, so large-tree searches abort immediately instead of running to completion.

Tests (tests/abort.test.ts, 4 new)

1. bash kills a running child on abort (<5s bound; measured ~450ms)
2. agent aborts a running tool and backfills [interrupted]
3. abort propagates through the sub-agent tool into the sub-agent's LLM call
4. pre-aborted signals make glob and grep reject immediately

All 28 tests pass (npm test); demo still runs end-to-end.