fix(tui): rank slash command prefixes

[code-yeongyu]

Summary

• rank ambiguous slash-command prefix matches so /sessio selects /sessions before /session
• keep exact /session matching ahead of longer commands
• move slash-command autocomplete ranking into a small TUI module with regression coverage

Root Cause

The editor autocomplete list showed both session and sessions, but the fuzzy result order highlighted session for /sessio. Pressing Enter accepted and submitted the highlighted singular command, so users saw session stats instead of opening the sessions HUD.

Validation

• red: node --test --import tsx test/autocomplete-slash.test.ts failed with actual: [ session, sessions ]
• green: node --test --import tsx test/autocomplete-slash.test.ts
• package coverage: node --test --import tsx test/autocomplete.test.ts test/autocomplete-slash.test.ts
• full check: npm run check
• build: npm --prefix packages/tui run build && npm --prefix packages/coding-agent run build
• manual tmux QA: typed /sessio; autocomplete selected sessions; Enter invoked session-observer instead of /session

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Summary by cubic

Fixes slash-command autocomplete ranking in packages/tui so typing /sessio selects sessions before session. Exact /session still ranks first.

• Bug Fixes

  • Rank longer prefix matches ahead of shorter commands when the prefix is ambiguous.
  • Preserve exact match priority.

• Refactors

  • Extracted ranking into packages/tui/src/slash-command-autocomplete.ts and wired into src/autocomplete.ts.
  • Added regression tests in packages/tui/test/autocomplete-slash.test.ts.

^{Written for commit 4ce014c. Summary will update on new commits. Review in cubic}