feat(verifier): add structured verification checklist and task completion enforcement (fixes #521)

[mickume]

Summary

• Adds agent_fox/spec/verification_checklist.py that auto-generates a structured verification checklist from tasks.md checkboxes, requirements.md acceptance criteria, and errata
• Injects the checklist into verifier context via assemble_context() (new archetype parameter), giving the verifier hard gates on task completion and requirement coverage
• Updates verifier profile with enforcement rules: unchecked subtasks without errata → FAIL, uncovered requirements → FAIL

Closes #521

Changes

File	Change
agent_fox/spec/verification_checklist.py	New: Checklist builder (task audit + requirement-to-test mapping + errata check + markdown renderer)
agent_fox/session/context.py	Added archetype parameter to assemble_context(); injects checklist for verifier
agent_fox/engine/session_lifecycle.py	Passes archetype to assemble_context()
agent_fox/_templates/profiles/verifier.md	Updated with Verification Checklist section and hard gate enforcement rules
tests/unit/spec/test_verification_checklist.py	New: 16 tests for checklist builder
tests/unit/session/test_context_verifier.py	New: 6 tests for context injection
docs/memory.md	Updated with session notes

Tests

• test_verification_checklist.py: task checkbox audit, requirement-to-test mapping, errata detection, markdown rendering, edge cases
• test_context_verifier.py: checklist injection for verifier, exclusion for coder, graceful failure handling

Verification

• All existing tests pass: ✅ (4342 unchanged)
• New tests pass: ✅ (22 added, 4364 total)
• Linter / formatter: ✅
• No regressions: ✅

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