Fixes#29. When a skill is already registered, register_skill_dir()
returned None, which caused fix_skill() to incorrectly report a failure.
Now returns the existing SkillMeta instead of None when the skill_id
is already present in the registry, making register_skill_dir() truly
idempotent as its callers (fix_skill, _auto_register_skill_dirs) expect.
Per reviewer feedback: keep strict \byes\b / \bno\b word boundaries to
prevent false positives, but widen confirm/reject/skip to stem-style
\bconfirm\w*\b etc. so common LLM variants like "confirmed", "rejected",
"skipping" still parse correctly instead of falling through to the
default False path.
