Tasks: record-feedback appends duplicate sibling headings when a gate is approved twice on one day

Task list

[x] 1. Regression test (red): two then three same-day recordings into one artifact in cli/tests/test_graph_integration.py
- Depends on: none
- Requirements: R1.1, R1.2, R1.3, R2.2, R3.1
- Test: T2 — uv run pytest cli/tests/test_graph_integration.py (red before task 2)
[x] 2. _unique_heading in cli/the_loop/graph/hooks/feedback.py; record_feedback uses it
- Depends on: 1
- Requirements: R1, R2.1
- Test: T2 green; T11 — markdownlint over the recorded artifact
[x] 3. Capability doc: the record-feedback rule and a changelog row in docs/capabilities/process-graph.md
- Depends on: 2
- Requirements: R1
- Test: T12 — make check