refactor(tool-timeout): extract createTimeoutController, deduplicate timer logic

[zxkjack123]

Stacked on PR to anomalyco/opencode anomalyco#36869 (tool-execution-timeout by @FahadBinHussain). This extracts the repeated AbortController + setTimeout + composeSignals + clearTimeout pattern into a single ToolTimeout.createTimeoutController() helper and adds timer-lifecycle tests.

What

• tool-timeout.ts: Add createTimeoutController({ tool, timeoutMs, userSignal }) — one-stop helper that creates the AbortController, arms the timer, composes signals, and returns { signal, dispose } for guaranteed cleanup
• tools.ts: Replace two duplicated blocks (built-in tools + MCP plugin tools) with createTimeoutController calls, reducing ~30 lines of duplicated low-level timer management
• tool-timeout.test.ts: Add 5 integration tests

New tests

Test	What it verifies
fires ToolTimeoutError after timeout	Timer actually fires with the correct error type
aborted signal has correct tool and timeoutMs	Error carries the right metadata
composes user signal with timeout signal	User abort (Esc) + timer abort combine into one signal
dispose clears the timer so no abort fires	finally { dispose() } prevents leak — timer is cleared before it fires
non-aborted signal when idle	Signal starts non-aborted, as expected

Verification

bun test test/session/tool-timeout.test.ts
  ✓ 15 pass / 0 fail

bun run typecheck (packages/opencode)
  ✓ tsgo --noEmit (clean)

The 3 pre-existing failures in task.test.ts (subagent description, variant, permission shape) are unchanged from upstream/dev — they depend on local .opencode/ agent config, not on this change.

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