rune v0.8.0

Multi-type support: skills, agents, rules.

- New ArtifactType enum drives per-type sections in manifest/lockfile
- -t/--type flag on add/remove/push/browse (auto-detect where possible)
- Per-project [paths] override for targeting other tools (Cursor, Pi)
- AGENTS.md generator emits separate <agent> tags for agentskills.io interop
- 62 tests (22 unit + 40 integration)