feat: add OpenCode adapter

[DietrichGebert]

Adds OpenCode (sst/opencode) as a host adapter, in keeping with the repo's agent-portability model.

What it does

• .opencode/plugins/ponytail.mjs: server plugin. Injects the ponytail ruleset into every chat via the experimental.chat.system.transform hook at the active intensity, and persists /ponytail <level> switches via command.execute.before.
• Reuses the shared instruction builder (hooks/ponytail-instructions.js): no copied SKILL.md, no duplicated filter/fallback logic, no edits to the shared hooks. Stays within the "keep adapters thin" rule in docs/agent-portability.md.
• .opencode/command/{ponytail,ponytail-review}.md: slash commands.
• tests/opencode-plugin.test.js: smoke test (inject-at-mode, switch-follows, off-suppresses, ignore-other-commands).
• Example opencode.json, plus README / agent-portability / help-card docs.

Verified against OpenCode 1.17.4

• Plugin loads cleanly (opencode debug config, no errors).
• Commands load into resolved config.
• The experimental.chat.system.transform hook fires and the injected system prompt reaches the model: Claude Sonnet 4.6 quoted the injected PONYTAIL MODE ACTIVE header; with mode off it correctly saw nothing.

Relationship to #15

Supersedes #15. That PR's plugin is built on OpenCode hooks that don't exist in the current API (session.created, tui.prompt.append) and mutates a non-existent output.systemPrompt, so it would silently no-op. The real injection point is experimental.chat.system.transform. This is rebuilt on the verified API and slimmed to reuse the shared builder (71 LOC of adapter vs 319). Thanks to @gailingmic for raising OpenCode support.

🤖 Generated with Claude Code

[vasilvestre]

I'm not sure about how to install this, I should download the repo locally ? Add it to my .config/opencode/ ?
Sorry 😓

[JulienBernard]

Same difficulty than @vasilvestre , i try everything but I do not understand how to setup Ponytail under my Opencode configuration.

1 - Clone this repo inside ~/.config/opencode/tools/ponytail
2 - Take ponytail.mjs from it and copy it to ~/.config/opencode/plugins/ponytail.mjs
3 - Update ~/.config/opencode/opencode.json to add it:

  "plugin": [
    "./plugins/ponytail.mjs"
  ]

4 - Start opencode
5 - No commands /ponytail available, no Ponytail skills visibles when I list them with /skills

The only working way is to clone the repo and start Opencode from it, but what the point if it's no available under our projects folders?

Edit: tried with the full path of ponytail.mjs into plugin, same issues, nothing happens.

Final Edit: fix PR incoming => #143