Steer active agent turns from follow-up messages

[boudra]

Linked issue

None

Type of change

• Bug fix
• New feature (with prior issue + design alignment)
• Refactor / code improvement
• Docs

What does this PR do

Follow-up messages sent while an agent is already running now steer the active turn when the provider supports it, instead of always replacing the run. Claude follow-ups are pushed into the existing input stream with next priority, Codex app-server follow-ups call the turn steering endpoint, and unsupported providers keep the existing replace behavior.

The shared send path now centralizes out-of-band command handling, unarchive/load behavior, user-message recording, and run startup so MCP, CLI, and websocket sends use the same policy. The app also marks local steering messages with a small “Steering conversation” hint while the daemon catches up.

How did you verify it

• npm install
• npm run build:daemon
• npm run typecheck
• npm run lint
• npm run format

CI is being watched by the existing babysit schedule for PR #781.

Extra review surface: this touches active-run concurrency and the client/server stream shape. The new message field is optional for old clients, but it is worth smoke-checking one active Claude or Codex follow-up from the app before merge.

Checklist

• One focused change. Unrelated cleanups split out.
• npm run typecheck passes
• npm run lint passes
• npm run format ran (Biome)
• UI changes include screenshots or video for every affected platform
• Tests added or updated where it made sense