Give the terminal an identity of its own, and clear the dead lint

[huyedits]

The TUI's own docstring called it "Hermes-shaped" while seven places in the code still wore Claude Code's orange (#d97757) — the exact copy-another-product mistake symbio_desktop/DESIGN.md records for the desktop window, which wore Claude's cream and orange until 2026-09-27.

Both halves are now deliberate. The SHAPE is Hermes Agent's and the terminal says so: MARK is the caduceus, GOLD is #ffd700 (Hermes Agent's own gold, and exactly xterm colour 220, so it renders honestly on a 256-colour terminal as well as a truecolour one). The GROUND stays Symbio's, so the window is not a copy of the thing it is modelled on — pond teal and tabby ink, read off the desktop's own :root tokens.

• symbio_tui/theme.py — new. The palette in one place, with the reasoning for the split written down next to the values.
• symbio_tui/app.py, widgets.py, faces.py — all seven #d97757 replaced; the banner now leads with the caduceus; the two "Claude Code" docstrings corrected to say Hermes Agent, which is what the code actually borrows.
• symbio/app/chat_style.py — _ACCENT moved from xterm 173 to 220 (#ffd700) and the caduceus added to the welcome frame and the name row. ANSI-256, hex and CSS are three representations for three media, so the value is repeated, not the meaning.

Verified by running it: the Textual stylesheet parses under textual 8.2.8 (CSS length 951, four gold occurrences, zero orange), the narrow-width banner ladder returns the face alone below 12 columns, and the CLI welcome panel renders the frame in 38;5;220 with the caduceus inside it.

Separately, the genuine lint findings rather than the cosmetic ones:

• chat_tools.py — how was assigned "pressed" and "clicked" and never read a single time. Deleted; it was doing nothing.
• chat.py — except Exception as e: bound a name it never used.
• chat.py — the stream default was a lambda assigned in the call site; it is now _default_stream_chunk, so a traceback in a stream write has something to point at.
• Plus 33 F541 (f-strings with no placeholder), the E401 one-line import in ops_eval.py, and two duplicate imports in tests.

NOT touched, deliberately: the 134 F401 "unused imports". Most are load bearing — symbio/init.py's are the public API, and chat.web, chat.sandbox and chat.training are monkeypatch anchors in tests/test_gui_launch.py and tests/test_sandbox_remote.py. Ruff cannot see a reference that only exists in another file's test. Same for the 48 E402: deliberate, post-sys.path and inside a try/except for the optional root tag_rag module.