tui: minor correctness/perf fixes (Etap 5) ...

The last batch from the code review — real bugs and edge cases, not cosmetics:

- tui_app._resolve_session: read session_id with .get and return None when a
  malformed server response omits it, instead of KeyError slipping past the
  api-call except (5.B3).
- cli /switch: surface the original error cause (404 vs network vs server) on
  a non-unique prefix match instead of swallowing it into a generic "not found"
  (6.B2).
- input_box _complete_command: a bare "/" with no hint list available no longer
  auto-picks the first command in the registry (3.B3).
- filesystem _render_grep: when the grep ran in regex mode, highlight with the
  actual regex so marked spans match what the server found — a literal highlight
  of a regex pattern would mark nothing (2.B2).
- file_refs _collect_files: filter sensitive entries out in the generator before
  sorted() so a huge directory is not fully materialized only to be mostly
  discarded (5.P1).
- renderers/tool ToolResultRenderer: cap a generic tool result at 200 lines
  (tail + marker) so a non-filesystem tool with huge output does not flood the
  chat bubble — filesystem has its own renderer (2.P3).

Tests: malformed-response /switch error surfacing, bare-slash no-auto-pick,
grep regex highlight, sensitive-subdir skip, tool-result truncation. Full
suite: 953 passed, 1 skipped.

tui: async REST api client — stop blocking the event loop (Etap 3) ...

The terminal client's REST helpers were synchronous (httpx.Client); every
caller inside the Textual event loop (attach/switch/resume/profile/stop) blocked
the whole TUI for the duration of each network round-trip — the spinner froze,
input stopped responding, the screen did not repaint. Move the network layer to
async and a shared pooled client:

- api.py: all 7 helpers are now `async def` over a single lazily-created
  httpx.AsyncClient (connection pooling) — awaits yield the loop while a request
  is in flight, so the TUI never blocks on the network.
- tui_app.py + commands/builtin.py: every `api.*` call site is now `await`-ed
  (all already ran in async workers / execute). _stop_stream_worker drops the
  dead `iscoroutine` check (api.stop_session is async, the await is real).
- cli.py: _resolve_session_id and _handle_command await api.*; _run_raw wraps
  the resolve + run in asyncio.run(_run_raw_async) so the sync click entrypoint
  stays unchanged. Removed the now-unused _run_one_shot wrapper.
- sessions_picker.on_mount: list_sessions runs in a worker (run_worker) instead
  of blocking on_mount — a slow/unreachable backend no longer hangs the modal;
  the input is focusable immediately and the list populates when the request
  returns.

Tests: every api mock across 6 client test files is now an `async def` fake
(test_tui_app, test_chat_panel, test_sessions_picker, test_terminal_client,
test_tui_export, test_input_box); the sessions picker tests use an async
_raise_not_found helper for the 404 path. Full suite: 939 passed, 1 skipped.

Navi Code TUI: interactive slash-command hints (Up/Down + Enter) ...

Make the inline command hints navigable:
- Up/Down move a highlight through the matching commands (wraps around).
- Enter while a hint is open runs the highlighted command (routed through
  UserSubmitted as /<name>, so the app's _run_command handles it — not sent
  to the agent). Enter with hints closed still submits the raw text.
- Tab completes the input to the highlighted command's canonical name and
  stays in the field for typing args (was: always the first match).

CommandHints stays a non-focusable Static (renders the highlighted line with
reverse video); _PromptInput owns all key handling via a sibling reference.

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

Navi Code TUI: inline slash-command hints + Tab completion ...

Show a non-interactive list of matching commands above the input box while
the user types a '/'-command (no whitespace yet). Tab completes the prefix to
the canonical name of the top match. Enter already routed '/...' through
_run_command, so submitting a typed command executes it instead of sending
it to the agent — unchanged.

- CommandRegistry.match(prefix): case-insensitive prefix match on name and
  aliases, exact match sorted first; empty prefix returns all.
- CommandHints(Static): purely visual, never takes focus, hidden by default.
- InputBox composes hints above the prompt and refreshes them on
  TextArea.Changed.

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