feat: slash-command dropdown with a themed panel

[Mouhand-Kaddo]

What

Typing / at the start of the input opens a dropdown listing every registered command (built-ins + skill commands), prefix-filtered case-insensitively and in alphabetical order. /mod narrows to mode, model, model-subagent, models, models-subagent; /MOD matches the same as /mod.

Behavior

• Open: / at input start (only there — not mid-message). Editing the command prefix reopens; a space after the command name closes it.
• Navigate: Up/Down move through matches and wrap at both ends.
• Select: Enter or Tab fills /name into the input without executing; a second Enter submits. Existing arguments are preserved.
• Dismiss: Escape closes the menu and restores the typed text. The inert No matching commands row (shown when nothing matches) stays dismissed until the prefix changes or Tab reopens it; Enter still submits normally there.
• Scope: @ files/agents, . personas, and path completion keep working. /-tokens at buffer start belong to the slash picker; mid-message absolute paths (read /etc/ho) still complete as paths.

Look

Themed in-flow panel below the input (no cursor-following float): dark purple background, thin muted border, lavender command names, muted descriptions, violet selection with a > marker, commands heading with a live match count, and a key-hints footer. Rows size to the number of matches, capped at eight, with scrolling. Colors come from the fixed theme (themes.py), not inline hex.

Implementation notes

• Slash candidates switched from fuzzy to case-insensitive prefix matching (prefix_matches); candidate list extracted to command_candidates (shared by the completer and the no-match row).
• The themed panel renders from the buffer's completion state; the stock CompletionsMenu float keeps serving the @/./path pickers, with a dynamically reserved spacer so it is not clipped (the app renders at natural height, full_screen=False).
• Approval Escape keeps precedence (completion Escape is filtered on ~approval_pending).

Tests

• Picker filter (case-insensitive, alphabetical, no fuzzy gaps) and path-token guard (buffer-start / vs mid-message absolute paths).
• Keyboard: menu opens on /, filtering, Enter fills then second Enter submits, arrows wrap, Tab accepts, Escape restores and reopens, space closes, no-match row dismissal/reopen, mid-message no trigger.
• Real-terminal (pty) coverage: panel rendering, colors, alignment, compact sizing, eight-row cap, scrolling, and Enter submitting normally on zero matches.

Full suite green locally (1098 passed); only pre-existing telemetry failures remain (missing optional opentelemetry/sentry_sdk extras). ruff check and ruff format --check clean.