Workflow round: review, PR loop, composer, compaction, real terminal #1493

This round fills gaps in what users do every day: starting work, steering, reviewing, and landing changes.

A. Keep context when the provider changes (bug, #237). setProvider clears sessionId but never sets replayContext (electron/services-threads.js:551), so a manual provider switch, or an automatic quota failover (runner-quota-wait.js), starts the next run with no history.

B. Composer

• Drafts, attachments and paste cards persist across reload and restart; today they live only in memory in composerSession.ts.
• ↑ in an empty composer recalls the last prompt.
• A send-key preference.
• Review comments from Changes and turn diffs become draft chips that are sent together as one prompt, instead of starting one run per comment.

C. Diffs

• A scope toggle in Changes: Uncommitted / Whole branch / This turn. Today it shows only diff HEAD, so committed agent work drops out of review.
• Syntax highlighting.
• Per-file lazy patches instead of a 100 KB cut-off.
• Split, wrap and ignore-whitespace toggles.

D. PR loop

• Generate the PR title and body.
• Merge-method choice (today it is hardcoded to --squash) and an option to auto-merge when checks pass.
• Watch-and-wake: a failed check (with the log tail), a changes-requested review, or a new conflict sends the thread one follow-up turn. Debounced, capped, and the watch can be stopped.

E. Native compaction (#318). /compact uses the provider's own compaction instead of forking. The compaction shows as a transcript event, and the context ring stays correct after the provider auto-compacts.

F. Real terminal (#1184, #1183)

• A PTY with xterm instead of pipes, so prompts, Ctrl-C and full-screen tools work.
• Splits.
• Scrollback that survives a restart.

Workers run in parallel on disjoint files; C starts after B lands.