fix(grep): hint when a zero-result literal search looks like a regex

[RioPlay]

Summary

Root-causes and fixes a silent false-negative in aden grep that I tripped over while auditing docs (it produced a wrong "aden ready doesn't exist" conclusion).

What happened: aden grep matches literal substrings by default; regex is opt-in (regex=true). A pattern like Ready|ready passed without that flag is searched literally — pipe included — and matches nothing. A human sees the empty terminal and self-corrects; an agent consuming {"total": 0} concludes the term is absent and acts on it.

Confirmed with a controlled experiment:

Query	Result
"Ready|ready", regex:false (default)	0 hits
"Ready|ready", regex:true	173 hits
"ready", ignore_case:true	126 hits

The fix keeps literal-default semantics (auto-promoting to regex would break literal searches for code like a|b or foo()), but stops the silent miss: when a search returns zero results in literal mode and the pattern carries regex metacharacters (| [ ( \ .* .+), emit a hint to retry with regex=true (or ignore_case=true). Surfaced in both human output and the JSON envelope as a new hint field — present only when firing, so successful searches are unchanged and existing consumers are unaffected.

looks_like_regex deliberately skips bare . * + ? (too common in literal code searches to be a reliable signal). Unit tests cover both the flagged idioms and the literals that must stay quiet.

Verified end-to-end on the built binary:

• zero-result literal alternation → total:0 + hint
• same pattern --regex → 8 matches, no hint
• normal literal hit → 14 matches, no hint field

Known follow-up (not in this PR)

Separately noticed: the [PATH] argument scopes only project-root discovery, not the search — grep <pat> some/file.rs still searches the whole project (and can dump megabytes when it matches minified asset lines). Filed for its own investigation; not touched here.

Test plan

• cargo test -p aden-cli green (new flags_regex_idioms / ignores_plain_literals pass)
• cargo clippy --workspace clean
• cargo fmt --all applied
• Manual end-to-end on the built binary (above)

License checklist

• No dependency changes — license checklist not applicable