feat(chat): cancel + block_id routing + replay + tests (0.8.0)

[tbitcs]

Summary

PR-2 of the three-PR chat-system completeness work. This adds four properties to the chat surface so the extension can power real multi-turn / multi-session work end-to-end:

1. Cancel button in the chat panel footer — issues SIGTERM (taskkill on Windows; SIGTERM → SIGKILL after 2s on POSIX) to the running CLI child. The panel-dispose hook also kills every running child, so closing the panel no longer leaves orphan specsmith chat processes.
2. block_id-keyed decision routing via the new ChatSessionRegistry — sessions are indexed by id and every tool_request / block_start kind=diff block_id is tagged with the session that emitted it. Decisions arriving from the panel route deterministically to the child that actually owns that block_id, so concurrent sessions no longer clobber each other's stdin.
3. Per-session event persistence + replay via the new ChatHistory module — every header / event / exit record is appended as JSONL to <projectDir>/.specsmith/sessions/<id>/events.jsonl. The first time the panel is opened against a project that has prior sessions, the most-recent one is replayed under a replay: <id> divider. Empty-utterance invocations of specsmith.openChat reuse this path to surface history without starting a new turn.
4. Mocha tests — 18 new unit tests across src/test/chat-session-registry.test.ts (routing semantics, observe/forget, fallback rules) and src/test/chat-history.test.ts (round-trip persistence, JSONL parsing, session listing).

The README also gets a new "Voice input (Web Speech API)" section explaining mic-permission expectations and the graceful fallback to text input when the API is unavailable.

Files

• src/ChatSessionRegistry.ts (new) — pure routing logic; no vscode / child_process imports.
• src/ChatHistory.ts (new) — JSONL append / read / list; pure stdlib.
• src/ChatPanel.ts — Cancel button, replay(), setRunning(), programmatic dispose(), replay-divider style and running / replay_divider inbound messages.
• src/ChatStreamConsumer.ts — rewritten around the registry + history modules; legacy _running: Map<projectDir, RunningSession> removed.
• src/test/chat-session-registry.test.ts (new), src/test/chat-history.test.ts (new).
• package.json — version bump to 0.8.0.
• package-lock.json — synced to 0.8.0 / engines ^1.116.0.
• CHANGELOG.md, README.md — entries for the new behaviour and the voice-input docs.

Verification

• npm run lint (tsc --noEmit): clean.
• npm run build (esbuild): 241.7kb bundle.
• npm test: 41 passing (was 23 before the new tests).

Artifacts

• Conversation: https://app.warp.dev/conversation/6f8aa790-049b-4ddf-9c52-4840728faee5
• Warp Agent Implementation Plan
• specsmith → 1.0 + Warp-level Agentic Workbench: Full Roadmap

Co-Authored-By: Oz oz-agent@warp.dev