fix(grep): scope the search to the PATH argument

[RioPlay]

Summary

Fixes the grep PATH-scoping bug I flagged in #47 and reproduced while testing the installed CLI.

aden grep <pat> <PATH> used PATH only to locate the project root, then always discovered files from that root. So grep <pat> some/file.rs silently searched the whole repo — scoping to one file returned 787 hits across 8 files including a vendored 3d-force-graph.min.js, and could dump megabytes when it matched minified asset lines.

Now discovery is scoped to PATH:

• a file searches just that file,
• a subdirectory searches under it,
• the project root (.) searches everything (unchanged).

Symbol attribution still keys off the project root, so hits keep their enclosing-symbol tags.

Implementation: discover_source_files is now a thin wrapper over a new discover_source_files_scoped(scope, root) (ignore rules still resolve relative to root). Single-file PATH is handled directly in cmd_grep.

Verified end-to-end on the built binary:

• grep "fn " crates/aden-cli/src/main.rs → 2 hits in 1 file (was 787 across 8)
• grep "fn " crates/aden-store → 87 hits, all under aden-store
• grep "fn " . → 10184 hits, repo-wide (unchanged)

Test plan

• cargo test -p aden-cli green (new discover_source_files_scoped_narrows_to_subtree)
• cargo clippy --workspace clean
• cargo fmt --all applied
• Manual end-to-end (above)

License checklist

• No dependency changes — license checklist not applicable