Wire SkillService to use SkillRepository for database persistence,
fixing the bug where `agents skill add` stored skills only in an
in-memory OrderedDict that was lost when the CLI process exited.
Changes:
- SkillService now accepts optional skill_repo and session_factory
parameters. When provided, add_skill() persists to the database
and the constructor pre-loads existing skills from DB rows.
- _get_skill_service() in the CLI now creates a DB-backed service
following the same engine/sessionmaker/repository pattern used by
the tool CLI (tool.py).
- _reset_skill_service() now installs a fresh in-memory SkillService
(instead of setting None) to avoid DB side-effects during unit
testing with parallel runners.
- remove_skill() also persists the deletion to the database.
Test coverage:
- Behave BDD: features/skill_add_persist.feature (4 scenarios)
- Robot Framework: robot/skill_add_persist.robot (3 smoke tests)
- ASV benchmark: benchmarks/skill_add_persist_bench.py
ISSUES CLOSED: #620
