refactor: invert memory->skill via synthetic-entry provider registry

skill depends on memory's MemoryEntry type (Skill::as_memory_entry), so
skill is the higher layer. memory previously reached up into
skill::SkillRegistry to inject synthetic skill entries into retrieval
candidates, forming a back-edge.

Mirror the established registry pattern: memory exposes
register_synthetic_entry_provider(fn) + collect_synthetic_entries(), and
the skill->MemoryEntry adapter is registered at cli::startup::run. The
memory_tests integration test registers the same provider locally.

Removes the memory->skill edge (memory now has a single in-cycle
out-edge, ->sidecar). cargo check clean; 24 memory lib tests pass.