Oh My Pi (omp) as a built-in ACP provider

[omnificate]

T3 Code has no plugin surface, and omp (can1357/oh-my-pi, 30k stars) ships a native stdio ACP server (omp acp), so the cheapest correct integration is a seventh built-in driver on the existing generic ACP client stack - exactly the shape of the Grok/Cursor drivers.

Prior art: PR #9038 implemented this and was closed unmerged in the Sept cleanup pass; issue #8985 was closed NOT_PLANNED with a pointer to Ideas. Opening this discussion per CONTRIBUTING.md before re-proposing the code.

What the driver does:

• spawn omp acp with RuntimeMode-mapped approval flags (Supervised --approval-mode=always-ask, Auto-accept edits --approval-mode=write, Auto --auto-approve, Full access --approval-mode=yolo)
• auth reuses credentials already under ~/.omp (ACP auth method 'agent'); T3 manages no keys
• model catalog discovered dynamically from the probe session's ACP configOptions (no hardcoded models); thought_level/context/fast configOptions map to reasoning/contextWindow/fastMode
• approvals via session/request_permission echoing advertised option ids; elicitation bridged for both the typed method and the official SDK's elicitation/create ext method
• off by default like cursor/grok/opencode; manual-only maintenance (T3 never guesses an omp update command)

Validation done on a local build: full server/contracts/web suites green (~8.5k tests incl. ~30 omp adapter tests for streaming, approvals, model-switch-without-respawn, cancel, subagent projection), plus a real end-to-end run against omp 18.1.15 (11.9k-model catalog, streamed turn in the built UI). Happy to open the PR rebased on current main if the direction is wanted.

[juliusmarminge]

Duplicate of the existing Ideas thread: #6814

Please fold the implementation details / PR offer from here into that discussion so the request stays in one place.