feat(engine): add state transition validation to GraphSync (fixes #523)

[mickume]

Summary

• Add VALID_TRANSITIONS table and _transition() validation method to GraphSync
• All mark_*() methods now validate state transitions (warn on invalid, never crash)
• Structured transition logging creates audit trail for debugging session failures

Closes #523

Changes

File	Change
agent_fox/engine/graph_sync.py	Added VALID_TRANSITIONS class constant, _transition() method with validation + structured logging, _transition_log in-memory audit trail. Updated mark_completed, mark_blocked, mark_in_progress, promote_deferred to route through _transition().
tests/unit/engine/test_sync.py	Added TestTransitionValidation (8 tests) and TestTransitionLogging (3 tests)
docs/memory.md	Updated with session summary

Tests

• TestTransitionValidation — valid transitions succeed, invalid transitions warn, terminal state enforced, unknown states don't crash
• TestTransitionLogging — audit trail recorded, cascade blocking logs all transitions, structured log messages emitted

Verification

• All existing tests pass: ✅ (4330 original)
• New tests pass: ✅ (12 added, 4342 total)
• Linter / formatter: ✅
• No regressions: ✅