run: disable per-step worktree snapshots and session diff summaries #146

Context

Each step of a headless run snapshots the whole worktree with git. Nothing in run reads the result.

• session/processor.ts:235,258,267,394 calls Snapshot.track() at start-step and finish-step, plus Snapshot.patch(). Each of these runs git add . over the whole worktree into the shadow repo at $DATA/snapshot/<projectId> (snapshot/index.ts). The first run hashes the entire repo.
• SessionSummary.summarize (session/prompt.ts:642, session/summary.ts) runs diffFull and writes session_diff to storage.
• cli/cmd/run.ts never subscribes to snapshot, patch or diff data.

Problem

That is several full-tree git add passes per step, which costs time on every step in large repos. It also writes shadow-repo data that is thrown away when the container exits.

Proposal

• In run, force snapshot: false. The guard already exists at snapshot/index.ts:53. Allow re-enabling it explicitly, e.g. AICTRL_ENABLE_SNAPSHOT=1 or config snapshot: true.
• Skip SessionSummary.summarize when snapshots are off.

Acceptance

• A headless run creates no $DATA/snapshot/* directory and runs no git add. Check with a test or by counting spawned processes.
• Patch parts and session_diff are still produced when snapshots are enabled explicitly.
• Before/after step timing on a large repo is recorded in the PR.