feat(cli): fuzzy command completer with alias + substring match

[emal-avala]

Summary

The slash-command tab-completer was prefix-only and name-only — typing /rvw, /github, or a command's alias returned nothing. Replaced with a scored match that surfaces the right command even when the user half-remembers the name.

Scoring

Tier	Score	Match
1	1000	Name has the partial as a prefix
2	500	Name contains the partial as a substring
3	150	An alias has the partial as a prefix
4	100	An alias contains the partial
5	~100	Name fuzzy-matches (each char in order, not necessarily contiguous)

Case-insensitive throughout. Stable-sorted by score desc, then alphabetically by command name for deterministic ordering at ties.

Display

Completions now include aliases when present:

/output-style — Set response style: ...  (alias: /style)
/install-github-app — Walk through `gh` CLI setup ...  (alias: /gh-setup)

New behaviours

/github     → /install-github-app     (substring match)
/style      → /output-style           (alias prefix match)
/rvw        → /review                 (fuzzy subsequence match)
/gh-        → /install-github-app     (alias prefix match)

What's unchanged

The inline-hint (grey autocomplete) behaviour is still prefix-only. Hints have to be literal continuations of what the user typed — you can't suffix a substring match mid-word without rearranging characters already on screen.

Test plan

• cargo fmt --all — clean
• cargo clippy --workspace --all-targets -- -D warnings — clean
• cargo test -p agent-code --bin agent completer_tests — 8/8 pass
  • empty_partial_matches_everything_at_top_score
  • prefix_beats_substring
  • substring_match_works
  • alias_match_is_recognised
  • fuzzy_subsequence_basic
  • case_insensitive_matching
  • no_match_returns_none
  • prefix_scores_higher_than_alias_prefix
• Manual: /gh<TAB> now lists /install-github-app; /STYLE<TAB> (uppercase) finds /output-style; /rvw<TAB> finds /review

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