_classify_removed_skills used naive 'in' substring matching to detect
whether a removed skill's name appeared in skill_manage arguments.
Short/common skill names (api, git, test, foo, etc.) matched
incorrectly when they appeared as substrings of longer words in file
paths (references/api-design.md) or content (latest, testing).
Replace with field-aware matching:
- file_path: needle must match a complete filename stem or directory
name, with -/_ normalised for variant tolerance
- content fields: word-boundary regex (\b) prevents embedding in
longer words
Also add 3 regression tests covering the false-positive scenarios.
744079ffe6