Add Devin CLI agent provider

[mjaggard]

Closes #608.

Changes

• Add Devin CLI to the engine picker, onboarding, voice spawn mapping and model catalogues, with SWE-2, curated direct models, four Fusion pairings and Devin's official installers.
• Launch an interactive Devin session with the hive briefing and the selected model/permission mode. Keep resume options ahead of the initial-prompt separator, and recognise existing agents saved as Custom with a devin command.
• Support the existing --prompt-file workflow by combining the user's file with the hive briefing in a private, Git-ignored per-agent file. The original file is not changed.
• Document the command-field setup and cover provider, installer, prompt-file and model behaviour in tests.

Before

Source-derived provider registry audit on origin/main: devin is inferred as custom, with no preset or model choices.

After

The same audit on this branch: Devin has its own preset and SWE-2 model choices.

Verification

• npm run typecheck — passed.
• npm run build — passed.
• Focused provider, prompt-file, model, installer and memory-ignore tests — 51 passed.
• Installed Devin CLI: --prompt-file plus inline -- prompt is rejected; file-only prompt returned READY; --resume plus prompt-file parsed and reached the expected missing-session error.
• All 13 explicit Devin model IDs in the baked and remote catalogues matched the installed CLI (devin models list --format json), including the four Fusion pairings with SWE-2 Medium.
• Human test (Matthew, 29 September 2026): ran this PR build locally with Devin as both the main god agent and a worker; reported that both worked perfectly. Shutdown was also part of the test: the separate Electron process exited cleanly with status 0, while the packaged app remained running. Individual Fusion pairings were not exercised in this test.
• npm run test:focused was interrupted after an unrelated PTY signal end-to-end test stopped progressing. The existing test/model-catalog-remote.test.cjs equality assertion also fails on current main: the remote catalogue already has newer non-Devin models than the baked catalogue. Both Devin catalogues agree.

[github-actions]

✅ Evidence received. Before and after are both attached. Thanks — this is what makes a PR reviewable in one pass.

[mjaggard]

@ chaitanyagiri this is now up-to-date and passing tests