feat(opencode): discover plugin package directories under plugins/

[licat2023]

Issue for this PR

Closes #33390

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

A local plugin listed in opencode.jsonc can point at a directory (one holding a package.json or an index.*) and resolves fine. But the zero-config plugins/ folder only scanned {plugin,plugins}/*.{ts,js}, so directory-style plugins were silently never discovered — you had to declare them in config.

This adds a second glob, {plugin,plugins}/*/{package.json,index.*}, and emits the directory URL (not the index file). The existing resolvePluginEntrypoint then resolves the real entrypoint via exports/main/index — the same path a config-declared directory plugin already takes, so there is no new resolution logic. The single * keeps it depth-1, so a package's own src/ or node_modules/ are never mistaken for plugins. INDEX_FILES is exported from plugin/shared.ts so detection and resolution share one list.

How did you verify your code works?

• bun typecheck in packages/opencode passes.
• Manual: dropped plugins/demo/ (a package.json with exports["./server"] + entrypoint) alongside a flat plugins/single.ts; both load and show up in the status dialog. A bare folder with no package.json/index.* is correctly ignored.

Screenshots / recordings

N/A — no UI change.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR