[Core] Add neutral session-state storage and migration

[teseo]

Kind

implement

Goal

Store session state by agent, surface, session, and cwd in a neutral location while preserving existing Claude state.

Non-goals

• Changing policy time windows.
• Adding host adapters.
• Deleting legacy state automatically.

Invariants

• Concurrent sessions do not overwrite each other.
• Stale-state handling remains deterministic.
• Legacy Claude state can be read during migration.
• Migration is idempotent and recoverable.

Behavioral contract / States

New sessions create a namespaced state record. Existing Claude state is read through a compatibility path and can be migrated once. Corrupt or stale records produce a safe diagnostic and never silently reset active work.

Scope surface

State helpers in Python/Node, hook/wrapper call sites, migration fixtures, and tests.

Acceptance

• Test two simultaneous sessions with different IDs and cwd values.
• Test stale, missing, corrupt, and legacy state records.
• Run: python3 -m pytest tests/test_guard.py -v, node --test tests/test_guard.mjs, and bash tests/test_install.sh.
• Provide a documented rollback procedure that leaves legacy Claude state intact.

Split triggers

• A host needs a state backend other than local files.
• Cross-process locking differs materially between runtimes.

Dependencies / Parent

Blocked by: #1, #2
Parent Epic: #15