fix(tui): prioritize exact alias matches in slash-command autocomplete

[IIzzaya]

When typing a prefix like /q in the composer, the slash-command autocomplete popup sorts results alphabetically by canonical command name. This means /clear (matched via the alias qingping) appears before /exit (matched via the exact alias q). The user who types /q almost certainly wants to quit — the match quality should drive ordering.

This PR replaces the alphabetical sort in slash_completion_hints with a three-tier priority sort:

• Priority 0: An alias exactly equals the prefix
• Priority 1: The canonical command
• Priority 2: An alias merely starts with the prefix

Within each tier, results fall back to alphabetical order. Skill entries and non-command completions (no CommandInfo) receive priority 3 and sort to the bottom.

After:

[gemini-code-assist]

Code Review

This pull request updates the slash_completion_hints function to prioritize command entries based on exact alias matches and prefixes. The feedback suggests optimizing the sorting logic by avoiding redundant string allocations and lookups within the comparison closure, recommending the use of &str and eq_ignore_ascii_case for better performance.

[Hmbown]

Thanks for the slash-command autocomplete fix. This was harvested into v0.8.40 release PR #1823 as commit 66b4a5f, and #1823 is now CI-green. Closing as superseded by the release branch; thank you for the focused UI polish.