Tasks: tiny-harness tui never asserts a participant, so it cannot send messages

Derived from bugfix.md, design.md and testing-plan.md. No approval gate follows this file.

Task list

• [x] 1. Regression test first: the real run_tui path sends a message
  • New scenario TUI sends a message under the asserted participant in tests/integration/embedded/test_cli_embedded.py: patch only HarnessApp.run_async to drive the app under Textual's pilot. Call commands.tui(settings, url=None, participant="alice"), type a message, expect COMPLETED and alice in the composer placeholder.
  • Rewrite the existing embedded scenario's stand-in to accept participant from the command and connect with it (R2.2).
  • Run it on today's code and record the red (no participant asserted, or the participant keyword the old signature rejects).
  • Depends on: none
  • Requirements: R2.1, R2.2
  • Test: T2 — uv run pytest tests/integration/embedded/test_cli_embedded.py (red)
• [x] 2. CLI: --participant and fail-closed resolution
  • Unit tests first in tests/unit/service/test_cli.py: parse --participant; tui_participant for flag, blank flag, OS default, OSError; main exits 2 naming --participant for a blank flag and for OSError.
  • Implement --participant, tui_participant, the refusal in main and the pass-through in dispatch.
  • Depends on: none
  • Requirements: R1.1, R1.3, R1.4, R1.5
  • Test: T1 — uv run pytest tests/unit/service/test_cli.py (red→green)
• [x] 3. Thread the participant through commands.tui and run_tui
  • Unit test first: commands.tui(settings, url="http://x", participant="alice") calls run_tui("http://x", participant="alice").
  • commands.tui takes participant and passes it on both paths; run_tui makes it a required str and drops the or "you" placeholder.
  • The composer placeholder reads Enter to send as <participant>; regenerate the TUI snapshots (uv run pytest tests/ui --snapshot-update) and check the diff is only that text.
  • Depends on: 1, 2
  • Requirements: R1.2, R1.6, R1.7
  • Test: T1 + T2 + T5 + T6 — task 1's scenarios go green; snapshots match
• [x] 4. Docs: capability doc, README and guide
  • docs/capabilities/surfaces-and-renderers.md: the TUI asserts --participant or the OS user name; history row for issue-20.
  • README.md, docs/guide/getting-started.md: mention --participant.
  • evidence/documentation.md.
  • Depends on: 3
  • Requirements: R1
  • Test: T12 — markdownlint via uv run pre-commit run --all-files

Dependency graph (DAG)

Checkpoints

• After 1: the red run is captured for evidence/integration.md.
• After 2 and 3: uv run pytest tests/unit tests/integration/embedded tests/ui.
• After 4: uv run pre-commit run --all-files; then the verification node runs testing-plan.md.

Review comments