Slash-command dropdown should open after whitespace, not only at start of prompt

Summary

The full slash-command dropdown (the multi-row two-column listing of every command with its description) only appears when / is the first character of the prompt. When / is typed mid-prompt, only a lighter inline suggestion shows — the rich dropdown is unreachable.

Repro

1. Open a Claude Code prompt.
2. Type /. Full dropdown appears with all commands + descriptions.
3. Clear, then type please run /. No full dropdown — only a lighter inline suggestion.

Expected

The full dropdown should also open mid-prompt, anywhere / is preceded by whitespace, a newline, or start-of-input. Same rendering, filtering, keybindings, and selection behavior as the start-of-prompt dropdown.

Trigger rule (precise)

Open the dropdown when:

1. Char at cursor's left is /.
2. Char immediately left of that / is one of: nothing (start-of-input), space, tab, or newline.
3. Normal prompt-input mode (not in a paste, modal, etc.).

Applies on any line of a multi-line prompt.

Filter query

The non-whitespace run between the triggering / and the cursor.
Example: please run /use-w| → query is use-w.

Edge cases (must NOT trigger)

• https://... — / preceded by :, not whitespace
• src/foo.ts — / preceded by c, not whitespace
• and/or — / preceded by d, not whitespace

Edge cases (MUST trigger)

• / at start of prompt (regression check)
• please /
• After Shift+Enter, on a new line: \n/
• Any later line of a multi-line prompt: ... /

Dismissal (mirror existing start-of-prompt behavior)

• Esc closes it
• Typing whitespace closes it (query run ends)
• Cursor moves out of the query run
• Backspace past the triggering /

Selection semantics

Selecting a command replaces /<partial query> with /<full command name> in place. Text before and after the cursor is preserved. The mid-prompt /command-name is not auto-invoked — invocation semantics stay unchanged (only start-of-prompt slashes invoke). This is purely a UI/discoverability change.

Why this should be small

The activation predicate for the existing start-of-prompt dropdown likely just needs to be broadened from cursor at col 0 AND char-left is / to char at trigger pos is / AND (trigger at col 0 OR char-left-of-trigger is whitespace). Rendering, filtering, keyboard handling, and selection are reused unchanged. The existing start-of-prompt case still satisfies the broadened predicate, so regression risk is low.