refactor(companion): remove app-specific server bridge - #5810

Summary

This replaces #5681, which GitHub closed during the repository transfer and fork-network separation. The branch has been rebuilt on the current dev history; the implementation scope is unchanged.

Remove the app-specific companion router, pairing flow, runtime package, and companion-only tests from core while preserving generic API tokens, chat/model APIs, webhooks, the responsive browser UI, PWA behavior, and unrelated 2FA QR support. Existing credentials are left to the normal operator token list/revocation path because stored API-token rows do not carry reliable companion provenance.

This is the final removal slice in #5674 and should merge only after the default-deny token capability boundary in #3149.

Depends on #5809, the focused implementation for #3149.

After that prerequisite merges, this branch must be rebased and its overlapping authentication and threat-model wording reconciled before merge, including removal of the now-retired Cookbook-scope reference.

Target branch

• This PR targets dev, not main.

Linked Issue

Fixes #5675

Part of #5674

Type of Change

• Bug fix (non-breaking — fixes a confirmed issue)
• New feature (non-breaking — adds new behaviour)
• Breaking change (removes the app-specific companion bridge and pairing contract)
• Refactor / cleanup (removes retired code)
• Documentation only
• CI / tooling / configuration

Checklist

• I searched open issues and open PRs and reviewed the full companion series.
• This PR targets dev.
• My changes are limited to the app-specific bridge/pairing surface and stale source references.
• I ran the full app interactively. This remains a draft until the prerequisite capability work and deployment-level verification are complete.

How to Test

1. Run python -m pytest -q tests/test_app_route_registration.py tests/test_api_token_user_route_gate.py tests/test_session_owner_attribution.py tests/test_api_chat_security.py tests/test_null_owner_gates.py. The final isolated focused run passes all 57 tests.
2. Run python -m pytest -q tests/test_task_scheduler_session_delivery.py. Both isolated scheduler regressions pass.
3. Confirm no /api/companion/* route is registered and the pairing endpoint is unavailable.
4. Confirm generic /api/v1/chat, /api/models, Codex/Claude, webhook, responsive web/PWA, API-token CRUD, and 2FA QR behavior remain registered and covered.
5. Run python -m pytest -q. The runtime-removal head completed with 4,633 passing and 5 skipped tests, versus 4,664 passing and 5 skipped on clean current dev; the 31-test difference was exactly the removed companion-only suite. The final review follow-up adds five passing generic regression tests without changing runtime code. Both full runs have the same 12 runner/baseline failures: the omitted .env.example, one offline URL-join check, two backup-recovery checks, and eight offline web-fetch checks.

Visual / UI changes — REQUIRED if you touched anything that renders

Not applicable. This removes an app-specific backend surface and tests; it does not change the responsive web UI.