fix(skill): circular symlinks in external skill dirs cause ENAMETOOLONG crash on Bun runtime

[litown]

Bug Description

On Bun runtime, opencode crashes with ENAMETOOLONG error during skill scanning when external skill directories (e.g. ~/.claude/skills/, ~/.agents/skills/) contain circular or broken symlinks.

Root Cause

The skill discovery system uses glob with follow: true to scan external skill directories. When circular symlinks exist (e.g., skills/finishing-a-development-branch/finishing-a-development-branch → points to itself), glob enters an infinite recursion. On Bun runtime, this causes path strings to exceed PATH_MAX, triggering ENAMETOOLONG from readdir which crashes the process. Node.js handles this gracefully, but Bun does not.

Additionally, broken symlinks (target deleted) cause unnecessary I/O errors during scanning.

Reproduction

1. Install a Claude Code plugin that creates skill symlinks (e.g., omc)
2. Some skills may leave behind circular or broken symlinks in ~/.claude/skills/
3. Run opencode with Bun runtime
4. Process crashes with ENAMETOOLONG during skill discovery

Environment

• Runtime: Bun 1.3+
• OS: macOS (confirmed), likely affects Linux too
• Affected directories: ~/.claude/skills/, ~/.agents/skills/

Proposed Fix

Pre-scan directories for circular and broken symlinks before glob traversal, and pass detected paths as ignore patterns to glob. Key changes:

1. packages/core/src/util/glob.ts: Expose ignore option in Glob.Options
2. packages/opencode/src/skill/index.ts: Add findBrokenSymlinks() that recursively detects circular symlinks (via readlink + path resolution + realpath visited set) and broken symlinks (via stat ENOENT/ELOOP), then passes results as glob ignore patterns

[github-actions]

This issue might be a duplicate of existing issues. Please check:

• Skill discovery follows symlinks into large directories, causing 120s+ startup on slow filesystems #27027: Same root cause — skill discovery uses followSymlinks: true with a ** glob in packages/opencode/src/skill/skill.ts, causing runaway traversal when skill directories contain symlinks. That issue covers the slow-startup variant; this issue covers the crash variant (circular symlinks → ENAMETOOLONG on Bun). They share the same fix surface and may be better addressed together.
• Glob tool returns zero results when broken symlinks exist in the search tree #24972: Related — broken symlinks in glob search trees causing silent failures; overlaps with the broken-symlink handling aspect of this report.

If you believe this is a distinct enough issue to track separately (crash vs. slowness), that's reasonable — just wanted to flag the overlap.