fix(core): discover local plugin packages - #41784

Closes #41530

What

Discover local plugin packages placed as immediate child directories under .opencode/plugin/ or .opencode/plugins/, matching the documented V2 local discovery behavior.

Before / After

Before: OpenCode scanned only direct .ts and .js children. A directory such as .opencode/plugins/reviewer/ was silently skipped even when package.json exposed "exports": "./entry.ts" or the directory contained index.ts / index.js.

After: OpenCode enumerates each discovery directory once and loads immediate child packages in deterministic code-unit order. It resolves the first existing contained entrypoint from string exports, module, main, index.ts, then index.js, while preserving direct-file and symlink discovery.

How

• packages/core/src/config/plugin/source.ts uses FSUtil.readDirectoryEntries to classify immediate files, directories, and symlinks without overlapping globs.
• Package metadata is schema-decoded, candidate entrypoints must remain inside their package directory after real-path resolution, and the first existing candidate wins.
• packages/core/test/config/plugin.test.ts covers string exports, fallback through missing exports to module, fallback through missing module to main, index.js, lexical traversal rejection, and symlink escape rejection.

Scope

• Does not recursively walk package contents or discover nested plugin packages.
• Does not add conditional/object exports or extensionless Node resolution; the documented and reported case is a string path to an existing entrypoint.
• Does not change explicit plugin configuration or npm package installation.

Testing

• bun run test test/config/plugin.test.ts from packages/core (15 passed)
• bun typecheck from packages/core
• bunx prettier --check packages/core/src/config/plugin/source.ts packages/core/test/config/plugin.test.ts
• git diff --check
• Push hook: bun turbo typecheck --concurrency=3 (33 tasks passed)
• Exact issue reproduction: started the worktree V2 server with isolated OPENCODE_CONFIG_DIR, queried /api/plugin, and observed both repro.direct-file and repro.directory-package after catalog initialization