feat(v2): make worktree branch naming configurable

[juliusmarminge]

Automatically generated worktree branches currently have no configurable naming convention. Add three modes in Source Control settings: a static prefix defaulting to t3code/, a model-selected semantic prefix, or custom instructions for the complete name. Environment defaults and project overrides work across web, desktop, and mobile.

Semantic prefixes are generated in the same request as the name. Custom names preserve casing and punctuation, and a collision keeps the temporary branch rather than adding a suffix. This applies to v2 worktree launches; manual names, commit-created feature branches, and worktree directories retain their existing behavior.

Based on the v2 branch from #2829.

Verification

• 428 focused tests passed across settings, project inheritance, naming prompts, provider generation, launch behavior, and client settings.
• Follow-up review fixes: normalize invalid static prefixes and explicitly label mixed modes on mobile. Four new prefix regression cases pass.
• Three focused Git integration tests passed, including custom-name collision handling.
• Server, web, mobile, contracts, and shared package typechecks passed. Targeted lint passed with one existing warning; React Doctor reported maintainability warnings.
• Verified in the isolated Browser panel: all modes, saving a prefix and custom instructions, persistence after reload, retaining values when switching modes, and resetting project overrides to inherited defaults.
• Native mobile UI was typechecked but not exercised in a simulator.

Before

Actual base 9cbaf77f7f7, same isolated data, project scope, 1280×500 viewport, and scroll position.

After

Mode switching

https://gh-file-drop-api-prod-mi5fy3sowv63ufte.pinglabs.workers.dev/f/3ad597055056e576/43d4cddd-606d-4d5a-a2fc-3101a882e06e-ea15274a-3fbe-4fba-a119-f1d5e08cd27c-mp4.mp4

Model: GPT-6. Harness: Codex.

[macroscopeapp]

All clear

^{Posted via Macroscope — Effect Service Conventions}

[macroscopeapp]

Approvability

Verdict: Not approved

Macroscope's review found this PR not approvable — This PR adds configurable branch naming across settings, prompt generation, and background worktree renames, while changing existing installations to use the new t3code/ prefix by default. The changed product default and multi-surface runtime behavior warrant human review.

^{You can add or adjust custom eligibility rules. Learn more.}

[github-actions]

Thread transfer impact

✅ Thread transfer remains within every enforced ceiling.

ℹ️ No successful main baseline artifact is available yet. This run establishes the initial measurement.

Provider	Metric	Main baseline	This PR	Impact	PR ceiling
Codex	Total thread wire	—	4.9 KiB	—	6.8 KiB	✅
Codex	Thread snapshot wire	—	3.7 KiB	—	4.9 KiB	✅
Codex	Live turn WebSocket wire	—	1.2 KiB	—	2.0 KiB	✅
Codex	Live turn WebSocket decoded	—	20.4 KiB	—	29.3 KiB	✅
Codex	Live turn messages	—	2	—	8	✅
Claude	Total thread wire	—	4.9 KiB	—	6.8 KiB	✅
Claude	Thread snapshot wire	—	3.7 KiB	—	4.9 KiB	✅
Claude	Live turn WebSocket wire	—	1.2 KiB	—	2.0 KiB	✅
Claude	Live turn WebSocket decoded	—	20.8 KiB	—	29.3 KiB	✅
Claude	Live turn messages	—	2	—	8	✅

Baseline: unavailable · PR result: d66f7cf · Source CI: success

Scenario and decoded snapshot size

10 historical turns, 5 command tools per turn, 878.9 KiB retained MCP result per historical turn, and a 1.05 MiB retained result in the measured turn.

• Codex decoded thread snapshot: 106.1 KiB
• Claude decoded thread snapshot: 106.4 KiB

Updated in place by a trusted workflow. PR artifacts are strictly validated and never executed.