[BUG] Plugin-shipped output styles not registered in 2.1.246: force-for-plugin ignored and style unresolvable by name (repro of auto-closed #64914)

Preflight Checklist

• [x] I have searched existing issues; the identical #64914 was auto-closed as stale with "open a new issue if this is still relevant"
• [x] This is a single bug report
• [x] I am using the latest version of Claude Code

What's Wrong?

Output styles shipped in a plugin's output-styles/ directory are not registered. Neither activation path works:

1. force-for-plugin: true in the style's frontmatter does not apply the style when the plugin is enabled.
2. Setting "outputStyle": "<style name>" in ~/.claude/settings.json silently falls back to Default while the style file exists only inside the plugin.

A/B isolation with the identical file:

• Plugin-only: fresh sessions show no trace of the style in the system prompt (probed by asking the session to quote the style's section), under both activation paths above.
• After copying the same file to ~/.claude/output-styles/ with the same outputStyle setting: the style is injected correctly and the probe returns the full section.

User-scope loading works and the file is valid; plugin-shipped registration is the broken piece.

The plugin itself is loaded and healthy: skills and hooks from the same install work. The plugin is enabled at user scope via enabledPlugins, installed from a git-source marketplace, and the plugin cache holds the current version with the style file and frontmatter intact.

Note: the /config outputStyle= autocomplete listing only built-in styles is the separate cosmetic gap tracked in #85246 and is not the failure reported here.

What Should Happen?

Per the output styles documentation (the force-for-plugin frontmatter table entry): a style in an enabled plugin's output-styles/ directory should be registered, selectable by name, and applied automatically when force-for-plugin: true is set.

Steps to Reproduce

1. Create a plugin with .claude-plugin/plugin.json and output-styles/mystyle.md:

```markdown
---
name: MyStyle
description: Test style
keep-coding-instructions: true
force-for-plugin: true
---

# Marker Section

Test instructions.
```

1. Enable the plugin at user scope (enabledPlugins in ~/.claude/settings.json); confirm the plugin's skills and hooks load.
2. Start a fresh session and ask it to quote "Marker Section" from its system prompt: absent.
3. Add "outputStyle": "MyStyle" to ~/.claude/settings.json; fresh session: still absent.
4. Copy the identical file to ~/.claude/output-styles/; fresh session: the section is present.

Environment

• Claude Code 2.1.246 (VS Code extension), Linux
• Plugin installed from a git-source marketplace, enabled at user scope

References

• #64914: identical report, auto-closed by the stale bot without a fix
• #85246: /config outputStyle= autocomplete gap (separate, cosmetic)