feat(desktop): add Codex thread migration foundation

[huntharo]

Summary

• add shared contracts, IPC/preload/renderer API plumbing for cross-profile Codex thread migration
• extend the Codex App Server client and backend registry so destination CAS can fork from a source CAS-provided rollout path without PwrAgent reading Codex thread storage
• add a main-process ThreadMigrationService with captive source CAS clients, grouped source thread listing, fork/validate/archive-last run execution, and a guard that blocks Move before archive when profile-owned worktrees are detected
• add Settings -> Thread Management for source profile/thread selection and Move/Copy starts

Safety notes

• Source rollout paths stay main-process-only; the migration picker receives bounded profile/project/thread summaries.
• Move archives the source only after destination fork and replay validation.
• Profile-owned worktree Move is currently blocked before fork/archive until the relocation strategy lands.

Validation

• pnpm --filter @pwragent/shared typecheck && pnpm --filter @pwragent/desktop typecheck
• pnpm exec vitest run --config vitest.workspace.ts apps/desktop/src/main/tests/thread-migration-service.test.ts apps/desktop/src/main/tests/codex-client.test.ts apps/desktop/src/main/tests/backend-registry.test.ts -t "ThreadMigrationService|forks a Codex thread from a CAS-provided rollout path|passes a CAS-provided source rollout path when forking for migration"
• pnpm exec vitest run --config vitest.workspace.ts apps/desktop/src/renderer/src/features/settings/tests/settings-screen.test.tsx
• pnpm lint:codex-storage
• pnpm lint:colors
• pnpm lint:boundaries
• git diff --check