The todo tool only had set/view/update/clear. Adding a step that surfaced
mid-task meant either 'set' (which replaces the plan and resets every status
to pending) or 'replan' (1–3 LLM calls) — both heavy for a single new step, so
the agent usually skipped recording it. Add a cheap 'add' op:

- navi/tools/todo.py: op 'add' (tasks: list[str]) appends new _Task entries to
  the end of the existing plan in pending status; existing steps and their
  statuses are untouched. Requires an existing plan (points to 'set'
  otherwise). Schema enum + op description + tool description updated.
- system_prompt.txt: tell the agent to record new subtasks with 'todo add'
  right away (not held in head, not a replan); clarify the 'small todo edit'
  line — add via 'todo add', drop/merge/reorder via 'set' (re-apply statuses).
- renderers/todo.py: TodoStartedRenderer renders the add card
  ('→ todo · add (N)') listing the new pending steps, not a JSON dump.

Tests: add appends + preserves statuses / requires existing plan / requires
tasks; renderer add card + empty-tasks. Full suite: 1005 passed, 1 skipped.

Point fixes from the full TUI/terminal-client code review (docs/code_review_tui.md),
each with a covering test:

- command_palette: guard on_list_view_selected against IndexError when the
  filter is empty and the placeholder row is selected.
- cli /profile: read session_id (not the absent "id") from the server — the
  handler no longer KeyErrors.
- todo_list: escape task text/validation/index/milestone before interpolating
  into rich markup so a task like "fix [bug]" no longer breaks the line styling.
- builtin /help: drop bold markup tags from status content — StatusRenderer
  builds a literal Text() that would show the tags as literal square brackets.
- tui_app on_user_submitted: always enqueue when a bridge exists (the input loop
  buffers across reconnect) instead of dropping the message on connected=False.
- chat_model stream_start: also reset _current_thinking so a dropped turn (no
  thinking_end) does not glue the next turn's reasoning onto the old block.
- todo renderer update: coerce validation to str before .strip() so a non-string
  JSON value does not AttributeError.
- renderers: remove the unused ArtifactRenderer (no production path emits
  type=artifact — TUI shows files via the OS); keep the diff renderer test.

Full suite: 937 passed, 1 skipped.

The todo update call card (→ todo · #3 → done) now shows the text of the
step being changed, plus the validation when present. The LLM's update args
carry only the index, not the task text, so the text is read from the
current plan row before the tool runs and attached to ToolStarted.metadata
(client-rendering only, backward-compatible).

Backend:
- events.ToolStarted gains a metadata dict (mirrors ToolEvent) → to_wire.
- navi/tools/todo: step_text_for_update(index, ctx) resolves the step text
  via _sid/_uid (so sub-agent isolation holds), started_metadata_for_call
  wraps it for both emit sites.
- agent.py (parent) and subagent_runner.py (sub-agent) enrich ToolStarted
  via the shared helper before emitting.

Renderer:
- TodoStartedRenderer update card reads msg.metadata.step_text → shows the
  step text + validation; falls back to 'no validation' on history replay.
- Removed the step-text line from the result card (it now lives in the call
  card) — result card is back to the compact 'plan · X/N done' summary.

Co-Authored-By: Claude <noreply@anthropic.com>