feat(examples): TUI theme switcher

[Meirtz]

Stacks on #63 (the Agentix TUI). GitHub shows the cumulative diff until #63 merges; the net change here is just the theme switcher — eval_tui/app.py (~20 lines: _themes set up in on_mount, a t binding, action_cycle_theme) + one pilot test.

What

Adds a theme switcher to the TUI: t cycles a branded agentix theme (registered best-effort) plus always-available built-ins (tokyo-night, gruvbox, nord, dracula, textual-light), with a toast showing the active theme. Reliable across Textual versions — the branded theme registration is guarded, and cycling only includes themes confirmed present in available_themes.

Verification

• ✅ ruff check, headless run_test pilot (t changes app.theme) — 11 pilot tests green.