Add Aider as a build-pod assistant backend

[dev-dull]

Part of #99. Depends on the pluggable-assistant framework.

Wire Aider as a second build-pod backend, launched under ttyd like Claude Code is today.

Scope

• Install — add Aider to the build-pod image (pip install aider-chat), per the image-packaging decision from the framework issue.
• Launch under ttyd — entrypoint.sh aider branch: run Aider in the app dir with auto-confirm (--yes-always) so non-technical users aren't prompted, analogous to Claude's --dangerously-skip-permissions.
• Git reconciliation — Aider auto-commits by default, but SUS already autosaves/commits (entrypoint.sh _autosave_loop) and owns Save/Publish. Run Aider with --no-auto-commits (or otherwise reconcile) so the two commit paths don't fight; confirm Publish/Save still behave.
• Seed prompt (Seed initial build prompt for new apps (not existing-app sessions) #96) — map the one-shot seed to Aider's initial-message mechanism (--message), keeping it one-shot and gated to new apps exactly as the Claude path is.
• Guidance — provide the Aider equivalent of build-pod/CLAUDE.md (a CONVENTIONS.md / read file): non-technical users, preview pane on the right, never mention git, X-SUS-* personalization headers, Save/Publish buttons.
• Auto-runner/preview — confirm the existing port-3000 runner + preview iframe work unchanged with files Aider writes.

Acceptance

• With buildPod.assistant=aider and a valid model/endpoint (see LLM-config issue), creating a new app launches Aider, it builds to the seeded description, the preview serves it, and Save/Publish work.
• Reconnect does not re-fire the build (same one-shot guarantee as Seed initial build prompt for new apps (not existing-app sessions) #96).

Notes

• Local-model quality varies; a weak model may follow the guidance file poorly. Track expectations in docs rather than blocking on it.