E2 (P0): Make plan/state loading non-destructive

[agustif]

Parent: #206

Objective

Make plan/state loading fail-safe and recoverable, not destructive.

Success Criteria

• No load-time destructive coercion or automatic reset to empty state/plan.
• Explicit quarantine artifact for invalid payloads.
• Save path blocks persistence after unsafe normalization unless explicitly overridden.

[agustif]

Implementation started in draft PR #227 (branch: feat/e2-nondestructive-persistence). This PR now includes persistence safety errors, quarantine snapshots, unsafe-save blocking, and tests.

[agustif]

Hash refresh after history cleanup: E2 branch head is now 0674092 (TASKS.md removed from commit history). Functional scope and test evidence are unchanged.

[agustif]

Completed via implementation PR(s) in this program. Closing as done.