feat: add opencode plugin compatibility

[gailingmic]

Add .opencode/ directory with plugin, commands, and skill definitions. Update hooks and statusline scripts to detect opencode flag path (~/.config/opencode/.ponytail-active) alongside existing Claude Code and Codex paths.

[DietrichGebert]

Thanks for kicking off OpenCode support, @gailingmic. It prompted a proper dig into the OpenCode plugin API.

One blocker worth flagging: this plugin is built on hooks that don't exist in the current OpenCode API. session.created and tui.prompt.append aren't real OpenCode hooks, and there's no output.systemPrompt to mutate in a session hook, so as written it would load but silently no-op (nothing injected, no mode tracking).

The real mechanism (verified against OpenCode 1.17.4 source plus a live run):

• Inject via the experimental.chat.system.transform hook (mutate output.system: string[]).
• Server plugins export default async (input) => hooks.
• Plugins are discovered through the opencode.json "plugin" array, not a .opencode-plugin/plugin.json or a package.json block.

I've opened #16 with a version rebuilt on that API. It also reuses the shared instruction builder (hooks/ponytail-instructions.js) instead of copying SKILL.md and the filter/fallback logic, so there's one source of truth. End-to-end tested on 1.17.4: the hook fires and the ruleset reaches the model.

Credit to you for the idea and the nudge to support OpenCode 🙏

[DietrichGebert]

Closing in favor of #16, which is rebuilt on the real OpenCode plugin API and verified end-to-end. Thanks again @gailingmic for raising OpenCode support.