feat: add native opencode plugin (server + TUI)

[sdeonvacation]

What

Adds a native opencode plugin under plugins/caveman/.opencode-plugin/:

• server.ts — injects caveman system prompt on every turn via experimental.chat.system.transform. Reads per-session state (disabled list + level) from XDG_STATE_HOME/opencode/caveman.json. Tracks subagent sessions via bus events and auto-applies ultra for them.
• tui.ts — registers three TUI commands in the / command palette: enable, disable, set-level (lite/full/ultra) with a dialog picker.
• package.json — manifest with exports map pointing ./server → server.ts and ./tui → tui.ts.

Also updates README.md to surface opencode as a first-class supported agent (install table row, feature matrix column, dedicated details section, footnote).

Before / After

Before: opencode listed only in "Any other agent" section with generic npx skills install (skill only, no auto-activation).

After: opencode has its own plugin with auto-activation every session, per-session enable/disable/level switching via TUI commands, and auto-ultra for subagent sessions.

Structure

Mirrors the existing .codex-plugin/ convention:

plugins/caveman/
  .codex-plugin/       ← existing
  .opencode-plugin/    ← new
    package.json
    server.ts
    tui.ts

Notes

• experimental.chat.system.transform hook is marked experimental in opencode's API — same as other plugin authors use it today
• compress and caveman-help not included (compress requires local skill files; help is skill-based)
• Tested against the real opencode plugin loader (packages/opencode/src/plugin/shared.ts) — server/tui must be separate exports per loader constraint