fix(desktop): allow V1 and V2 to run concurrently

[juliusmarminge]

Opening V2 while V1/nightly is running can leave the app with no environments because both desktop processes open the same Chromium profile. In a macOS two-process reproduction, the second process fails to open t3code:connection-runtime with an IndexedDB backing-store lock error.

Packaged V2 now uses its own t3code-v2 profile and never falls back to the V1 profile. On Windows, the new profile imports only Local State once to preserve the encryption keys used by saved connection credentials. All profile selection and Windows initialization now live in DesktopUserData.resolveUserDataPath, shared by startup and the Clerk bridge. Development profile handling stays unchanged. This profile is Electron storage under the OS application-data directory; the server T3 home and its databases stay where they are.

Targets #2829.

Validation:

• 20 focused desktop tests pass. New coverage is limited to profile isolation and Windows credential preservation/error context.
• Targeted lint and desktop typecheck pass.
• Isolated macOS Electron reproduction: shared profile fails; separate V2 profile opens IndexedDB and commits a write while V1 stays running. This checks the storage failure directly, rather than launching the complete T3 app.

Before, shared profile:

After, separate V2 profile:

Browser cookies and caches do not migrate, so websites opened inside V2 may require signing in again.

Prepared with Codex.

[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: 7f1069e · 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.

[macroscopeapp]

Approvability

Verdict: Not approved

Macroscope's review found this PR not approvable — This PR introduces startup-time profile migration and changes the production Electron user-data default to t3code-v2, affecting concurrent operation, browser state, and Windows credential metadata. A reported fresh-install startup failure remains unresolved, so the cross-platform initialization behavior requires human validation.

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