Tasks: gate the brainstorm on its author's approval

Derived from bugfix.md, design.md and testing-plan.md. All the work is in this repository. Each code task is test-first: its named test goes red, then green.

Task list

[x] A1: absentOutcome on validate-artifacts. Add the parameter to the optional-node branch in cli/the_loop/graph/hooks/artifacts.py.
- Depends on: none
- Requirements: R2.1, R2.4
- Test: T2
[x] A2: the graph. Add brainstorm-approval to pdlc-work-item-loop.yaml, set absentOutcome: not-entered on brainstorming's exit, re-point the edges per the design's table, and add the node to spec-chain. Update the pinned tests (test_graph_skips.py, test_core_graphs.py) and the e2e scenario node lists.
- Depends on: A1
- Requirements: R1.1, R1.4, R1.5, R2.2, R2.3
- Test: T1, T5
[x] A3: the runtime walk. Add the T3/T4 integration tests that walk the gate.
- Depends on: A2
- Requirements: R1.2, R1.3, R1.4, R1.6, R2.1, R2.2, R2.3
- Test: T3, T4
[x] A4: the checklist fixtures. Regenerate or edit the Jira checklist fixtures and any selection test that pins the row list.
- Depends on: A2
- Requirements: R2.3
- Test: T6
[x] A5: the documentation. Update the files in the design's documentation table, including the brainstorm.md template, and docs/capabilities/spec-workflow.md.
- Depends on: A2
- Requirements: R3.1, R3.2
- Test: T9
[x] A6: verify. Run the testing plan's activities and record the evidence.
- Depends on: A3, A4, A5
- Requirements: all
- Test: T7, T8