Tasks: make the phase-selection comment easier to read

Task list

[x] A1: red tests. test_selection_layout.py (T1–T4) and the digest cases (T5), failing before the change. Req: R1–R3 · Test: T1–T5.
[x] A2: the renderer. Rewrite _checklist_body, _channel_lines and _choice_lines to the layout; rows unchanged. Req: R1, R2, R3.1–R3.3 · Test: T1–T4. Deps: A1.
[x] A3: the Slack digest. Unfold <details> in to_mrkdwn and condense. Req: R3.4 · Test: T5. Deps: A1.
[x] A4: existing tests. Update the assertions that named the old wording. Test: T6. Deps: A2.
[x] B1: the docs. The checklist example in docs/cli/commands/graph.md, the capability doc's criterion and history row. Deps: A2.
[x] C1: verify. Run the testing plan; commit the before and after renderings; record evidence/verification.md. Test: T6, T7. Deps: A2–A4.
[x] C2: review and ship. Self-review rounds, security review, documentation record, reviewer briefing, PR. Test: T8. Deps: B1, C1.