Make ,-prefixed internal command completion work reliably while typing on bub

[WSL0809]

In the interactive CLI, internal commands are prefixed with , (e.g. ,help, ,tape.info). Completion is currently implemented with prompt_toolkit’s WordCompleter. In day-to-day use, though, the suggestions don’t seem to stay active: candidates may appear right after
typing ,, but once you continue typing (e.g. ,h / ,hel) the list often disappears, and complete_while_typing doesn’t consistently keep it going.

As a result, the internal command completion experience can feel close to “no autocomplete”.

Steps to Reproduce

1. Run: uv run bub chat
2. Type , in the input box
3. Check whether a candidate list appears
4. Type any character, e.g. h to make it ,h (or continue to ,hel)
5. See whether the candidate list disappears immediately (and doesn’t automatically reappear)

Actual Behavior

• When typing ,, candidates may appear briefly
• After typing , and then any additional character (e.g. ,h / ,hel), the list often disappears and autocomplete stops showing suggestions
• It’s hard to rely on completion to discover or enter internal commands

Expected Behavior

• As long as the input starts with , and the cursor position is valid, internal command completion should remain available
• Completion should support prefix matching (e.g. ,hel → ,help, ,ta → ,tape.info)
• After pressing backspace, as long as the input still starts with ,, completion should remain available