v0.16.0 -- prompt-template + mcp artifact types

Adds first-class manifest support for two new artifact types
alongside skills:
- prompt-template (writes to .claude/prompt-templates/)
- mcp           (writes to .claude/mcps/)

Companion bug fix: commit_and_push now resolves the destination
directory per artifact type instead of hardcoding the skill path,
so pushes against prompt-template and mcp registries land
correctly.

Tracked at andunn/tanuki harness/pi-extension-self-doc and
nomograph follow-up work that depends on the new types.