Tasks: a Slack kickoff offers Do or Start, not Start alone

Four tasks, all in this repository and all small. The renderer comes first, because the two call sites render through it. The docs come last. Every code task is red → green: the test is written first and fails, then the code makes it pass.

Task list

[x] 1. Renderer: a Do button and the description lines
- slack.py: COMMAND_BUTTONS gains "do": "Do". New KICKOFF_COMMANDS = ("do", "start") and COMMAND_DESCRIPTIONS (fixed words, do and start only), both in __all__. render_reply_blocks inserts a description section (*<Label>* — <line>, one per rendered command that has a description, in button order) between the text section and the actions block.
- Depends on: none
- Requirements: R1.1, R1.2, R1.4, R1.5
- Test: T1 — tests/test_channels_buttons.py (two commands give text, then descriptions, then [Do, Start]; one command gives one line; none gives text only; execute has no line; the tables agree) (red→green)
[x] 2. Call sites: every kickoff path offers both
- inbound._open_work_item and commands._create_verb: command_buttons_for("start") → command_buttons_for(*KICKOFF_COMMANDS), and their comments updated.
- Depends on: 1
- Requirements: R1.1, R1.3, R1.4
- Test: T2 — tests/test_channels_integration.py (the existing kickoff reply test now expects [do, start]), tests/test_channels_commands.py (/the-loop new), tests/test_channels_kickoff_picker.py (the picker's opened reply) (red→green)
[x] 3. Scenarios: pressing Do, a disabled keyword, the abuse cases
- Integration tests with Gherkin docstrings and Requirement: links: pressing Do records an unmarked the-loop do that parse_command reads as do, and the reply is edited naming "Do"; routing.control.keywords.do: "" renders Start alone; an unlisted member's Do press records and edits nothing; a crafted non-keyword value on the Do action_id arms nothing; no control.command grant means no buttons at all.
- Depends on: 2
- Requirements: R1.4, R2.1, R2.3, R2.4, abuse 1, 2, 4
- Test: T2 + T8 — tests/test_channels_integration.py -k "do_press or do_button"
[x] 4. Docs and visual evidence
- docs/capabilities/channels.md: the rendering bullet (Execute / Start / Do, each with its line on the kickoff reply) and the kickoff and /the-loop new bullets, plus a history row for issue-529. The "Opened … Start button" wording in docs/guides / README, if any exists, is updated too.
- Screenshots of design/kickoff-reply.html in light and dark.
- Depends on: 3
- Requirements: all (docs ship with the change)
- Test: T5 + T12 — the screenshots; uv run pre-commit run --all-files