Summary

Four stacked PRs add regression coverage for the non-TUI tui: false path,
fix the defects it exposed, and document it as linear terminal mode.

Changes

• Linear-mode regression harness + three risk cases #688: adds an offline fake-LLM harness for _run_linear,
  echoes queued input, and handles empty non-streaming responses.
• Enforce the linear-output transcript invariant #689: enforces an append-only transcript invariant and
  disables both spinner renderers in linear mode.
• Linear-mode agent coverage and task-lifecycle fixes #690: covers MCP tools, confirmations, delegation,
  compaction, commands, and clean exit; tracks memorizer tasks with
  fire_and_forget, cancels background and sub-agent work at exit, and clears
  stale agent state.
• Document the canonical linear terminal mode #691: documents the canonical linear profile and
  dumb-terminal fallback.

Rationale

The offline harness exercises the real async path without provider or network
variability. The transcript invariant makes tui: false safe for logs, pipes,
scripts, and terminals that cannot support cursor movement; spinners are
disabled because they redraw existing cells.

Bare asyncio.create_task calls left memorizer work unowned at shutdown.
Using the existing fire_and_forget helper retains each task, captures its
failure, and registers it for cancellation. Sub-agent tasks and service state
have separate owners, so exit also cancels and clears them explicitly. The
documentation records these tested guarantees instead of leaving tui: false
as an accidental fallback.