fix: migrate the plugin to the OpenCode V2 plugin API

[miguelmartens]

Summary

Port the plugin to the OpenCode V2 plugin API. V1 keeps working through the legacy entry point the migration guide documents.

 .opencode/plugins/
-├── caveman.mjs                  # V1 function export — V2 rejects it on load
-└── caveman-frontmatter.cjs
+└── caveman/
+    ├── index.mjs                # default: { id, setup(ctx), server() }
+    └── caveman-frontmatter.cjs

The directory form is required: V2 only discovers local plugins as <dir>/index.*, not bare files.

 setup(ctx)                       # V2
+  ctx.command.transform          <- config hook (commands)
+  ctx.skill.transform            <- config hook (skills dir)
+  ctx.agent.transform            <- config hook (upsert; V2 has no agent add)
+  session.hook("context")        <- experimental.chat.system.transform
+  session.hook("prompt")         <- chat.message
+  command execute                <- command.execute.before + template submit
 server()                         # V1 1.18.29+, unchanged behavior

Evidence

• Before: opencode serve in a scratch project with the published @miguelmartens/opencode-caveman@1.0.2:
  failed to load plugin ... Plugin must export a default definition with an id and an effect or setup function.
• After: same package entrypoint with this change: plugin active; all 4 commands, 4 skills, and 3 cavecrew agents registered; /caveman ultra persisted the level; stop caveman turned it off.
• npm test 33/33 pass, npm run format:check clean.

Merge Danger

Door: two-way — reverting restores the V1 entrypoint.
Blast Radius: V1 below 1.18.29 no longer loads the package (function export becomes an object); 1.18.29+ works via server(). In V2, same-ID skills already present in ~/.agents/skills or project config override the bundled copies, because config sources apply after plugin transforms.