[lanly-dev]

What

Turns the @deval chat participant from a plain chat forwarder into a real
tool-calling agent, and captures every run for benchmarking.

Changes

• Agent loop (src/agent/loop.ts) — dependency-free core: model turns,
  tool dispatch with PreToolUse/PostToolUse pairing, 10-turn cap. Takes the
  model, tools, and capture sink as ports, so the same code runs in the
  extension host, in unit tests (fakes), and in benchmarks (scripted model).
• Tools (src/agent/tool-schemas.ts, src/agent/tools.ts) — three
  read-only tools (deval_readFile, deval_listFiles, deval_grep) declared
  in package.json (languageModelTools) and registered via vscode.lm,
  with workspace-escape guards and output caps. Read-only on purpose: no
  confirmation UX needed for v1.
• Adapter (src/agent/vscode-adapter.ts) — translates loop messages to
  vscode.LanguageModelChatMessages, streams text to the chat, executes tools
  via vscode.lm.invokeTool so the UI shows invocations.
• Capture writer (src/events/event-writer.ts) — writes each run to
  .deepeval/vscode-agent-events/<session-id>.jsonl in the format
  event-log.ts reads, so DeepEval: Evaluate Captured Local Agent Run
  scores the agent with no extra wiring.
• Participant (src/extension.ts) — runs the loop per request, reports
  progress, summarizes the run when done.
• Tests — 9 new unit tests (loop core, writer); new deterministic
  benchmark benchmarks/agent-loop-benchmark.test.ts (scripted model through
  observe(), ContainsAllMetric + ToolCorrectnessMetric, no API key).
• Docs — README (@deval agent section, repo layout) and CHANGELOG.

Verification

• npm run check-types — clean
• npm run lint — clean
• npm run benchmark — all suites pass, 100% metric pass rate (incl. the new
  agent-loop case)
• 9 new mocha unit tests — passing

Follow-ups (not in this PR)

• The old reference agent (todoAppAgent) and the scaffold command are now
  redundant with @deval as the system under test — candidates for removal.
• writeFile / runCommand tools behind user confirmation (v2).
• Extension-host tests for the new modules (xvfb needed; not run here).