Shared Readiness Decision for existing commands

[BramVR]

Parent

#70

What to build

Make status, tree, and agent prepare consume one Readiness Decision module for path, dirty source checkout, remote, fetch, base, policy, env, blocker, and warning semantics. Agent Prep should stop recomputing the same readiness rules and should consume the same handoff green decision as future doctor.

Acceptance criteria

• status and tree still report the same current states and diagnostics.
• agent prepare uses the shared Readiness Decision instead of duplicating path/Git/env checks.
• Dirty source checkout remains a warning for temp-clone handoff.
• Missing base, origin mismatch, invalid policy, fetch failure, path conflict, and blocking env remain blockers.
• Tests prove status, tree, and agent prepare share readiness semantics.

Blocked by

• Git Operations and Project identity locality #71