feat: switch to Caveman 3.2.0 modes (caveman, ultracave, megacave)

[miguelmartens]

Summary

Aligns the plugin with upstream Caveman 3.2.0: levels become modes, and the vendored rules move to the v3.2.0 tag.

 commands
-  /caveman lite|full|ultra|wenyan-lite|wenyan-full|wenyan-ultra
+  /caveman caveman|ultracave|megacave|off   (+ /caveman status)
+  /ultracave, /megacave

 state (~/.config/opencode/.caveman-active)
-  lite | full | ultra | wenyan-*
+  caveman | ultracave | megacave | off      (legacy names still resolve)

 injection
-  caveman skill filtered to the active level's table row + examples
+  active mode's skill loaded whole under `CAVEMAN MODE ACTIVE — mode: <mode>`

• Re-vendored skills and cavecrew agents from upstream tag v3.2.0 (NOTICE.md pin updated; only the documented model: line is dropped).
• Added /ultracave and /megacave commands; /caveman status reports the mode without switching.
• Ported hooks, plugin (V2 + V1), commands, tests, and docs to the mode API. Legacy lite/full/ultra/wenyan* resolve everywhere: config, state file, command args.
• README now carries a compatibility label for rules v3.2.0.

Evidence

• Before: suite red on the in-progress tree — ERR_MODULE_NOT_FOUND .../.opencode/plugins/caveman/index.mjs and readMode is not a function on the old API names.
  After: npm test → tests 35, pass 35, fail 0; npm run format:check → clean (both also enforced by CI matrix on Node 20/22/24).

Merge Danger

Door: two-way — revert PR; released npm version cannot be unpublished after the registry window.
Blast Radius: plugin users' command and state compatibility is preserved via aliases and state normalization; worst case a bad injection affects every session, so the merge should follow the green CI run.