tui: code-review Etap 1 — crash/injection/reconnect fixes + drop artifact renderer ...

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.

tui: unify syntax highlighting under Theme.code_theme; highlight filesystem read output ...

All syntax-highlighted code in the TUI now resolves one Pygments style per
theme from a single point: Theme.code_theme (gnexus-dark=dracula,
gnexus-light=paraiso-light). Previously the dracula/github-light name was
duplicated in artifact.py and markdown_content.py, and the light theme's
"github-light" is not a real Pygments style (only github-dark exists), so
rich silently fell back to "default" and light-mode code was never
highlighted — fixed by switching to paraiso-light.

New renderers/syntax.py::highlight_code() is the shared Syntax factory
(always theme.code_theme + background_color=theme.surface.hex); artifact.py
and the new filesystem read path build through it, and
_theme_aware_code_theme now reads Theme.code_theme via ThemeRegistry.

filesystem FilesystemToolResultRenderer._render_read: the file body is now
syntax-highlighted via highlight_code with the language guessed from the
read path (guess_language). The server's "{num:>width}: {line}" prefix is
stripped (_strip_number_prefix) so Syntax renders its own line-number column
and multi-line constructs (triple-quoted strings, block comments) highlight
correctly across line boundaries; numbered=False renders without numbers.
The header plaque (accent path) and the large-file warning stay as Text;
body is Group(header, [warning,] Text, Syntax). Unknown extensions fall back
to the "text" lexer (plain).

Diff content keeps marker-only coloring (combining Pygments token styles
with the +/- tint is non-trivial); grep/list/info/find unchanged.

Tests: tests/clients/test_code_theme.py pins the single-point contract
(markdown/artifact/read all resolve theme.code_theme, dark != light);
test_filesystem_renderer read tests rewritten for the Group+Syntax body,
plus language-guess and numbered=False coverage. Full suite: 932 passed.