docs: add the Cursor Tier 1 support plan

Cursor is the one agent where the distribution table in CLAUDE.md claims more
than caveman delivers: the table says always-on rule, but a Cursor user gets a
per-repo .mdc only via --with-init and the global skills install may not land.

Plan documents ground truth verified on this machine — the empty
~/.cursor/skills, the plugin cache that loaded caveman as an Agent Plugin (skills
and MCP only, hooks ignored), and the IDE transcript format that carries no
usage block — then maps each Cursor surface to its Claude Code counterpart.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>