Title: Fix discovery for plugin-bundled OpenCode skills

Summary:
OMO's custom skill loader currently discovers filesystem-backed OpenCode skills under config skill dirs, but misses skills bundled inside OpenCode plugins that are injected at runtime, such as superpowers/systematic-debugging.

Problem:

• superpowers follows the official OpenCode installation flow by registering an OpenCode plugin and exposing its skills from the package skills/ directory.
• OpenCode can use those skills, but OMO's skill tool only scans physical OpenCode skill directories and does not discover plugin-bundled skills from package roots.
• Result: users see false negatives like Skill or command "systematic-debugging" not found even though superpowers/systematic-debugging is installed and available.

What this changes:

• Extend OMO skill discovery to inspect OpenCode package roots under:
  • the OpenCode config node_modules
  • the OpenCode cache node_modules
• Treat a package as a plugin-bundled skill provider when it has:
  • .opencode/plugins/*
  • skills/
• Load those bundled skills with a package-based name prefix such as superpowers/...
• Keep the existing deduplication and priority behavior intact

Regression coverage:

• Adds a test that creates a fake plugin package in the OpenCode cache and verifies that discoverSkills({ includeClaudeCodePaths: false }) finds superpowers/systematic-debugging

Why this is safe:

• Discovery remains read-only
• Packages are only considered when they look like actual OpenCode plugins
• Namespacing uses the package basename, matching how users expect to reference plugin-bundled skills

Validation:

• bun test src/features/opencode-skill-loader/loader.test.ts
• Verified locally against superpowers that all namespaced skills load through OMO after discovery is fixed

Summary by cubic

Fixes OpenCode plugin skill discovery so OMO finds skills bundled in plugins (e.g., superpowers/systematic-debugging, @obra/superpowers/systematic-debugging) and stops “skill not found” errors. OMO now scans plugin package roots in config/cache node_modules and loads their skills/ with a namespaced prefix that preserves npm scopes.

• Bug Fixes
  • Scan OpenCode config and cache node_modules for plugin packages.
  • Detect plugins via .opencode/plugins/* JS/TS entries; load from skills/.
  • Prefix skill names with the package (including npm scope) to avoid collisions.
  • Integrate bundled skills into discoverSkills, discoverAllSkills, and loadOpencodeGlobalSkills with existing priority unchanged.
  • Add tests for cache discovery and scoped packages.

^{Written for commit 8e87ea6. Summary will update on new commits.}