fix(search): always check parent index regardless of cwd

[vlasky]

Summary

• Removes the is_external_project_path gate that skipped find_parent_index when the search path was not a subdirectory of cwd
• Fixes: searching an absolute path from an unrelated working directory failed to find the parent index, causing either a redundant index build or no results
• The local_index_exists check already short-circuits the common case; find_parent_index is cheap (one read_dir + one JSON parse per indexed project)

Repro

# Index a project
colgrep init ~/projects/myrepo

# Search a subdirectory from an unrelated cwd
cd /tmp
colgrep "some query" ~/projects/myrepo/src/

Expected: uses existing index for ~/projects/myrepo
Actual (before fix): fails to find the parent index because /tmp is not an ancestor of the search path

Test plan

• cargo build -p colgrep clean
• cargo clippy -p colgrep -- -D warnings clean
• cargo test -p colgrep --lib passes (662 tests)
• Manual: search a subdirectory of an indexed project from an unrelated cwd, confirm parent index is used

[raphaelsty]

Amazing MR @vlasky you rock 🤘☺️