tmux-resurrect integration + concurrency warning polish (Session #2)

[JRedeker]

Summary

Add manual tmux-resurrect support for session persistence across tmux server restarts, and polish concurrency warning when multiple sessions access the same project.

Why

tmux server restarts destroy all sessions. Resurrect restores them. Concurrency warnings prevent state corruption from parallel sessions.

Acceptance Criteria

• oca session resurrect restores sessions from saved state
• Warning emitted when second session opens on same project
• Warning includes project name and existing session info
• Works with Pattern B topology

Dependencies

Blocked by Session #0 (Pattern B topology).

Proposal

docs/proposals/2026-05-03-session-and-resource-architecture.md § Change #2

WSJF Estimate

Value: 5 | TC: 3 | RROE: 4 | Effort: 3 | WSJF: 1.7