fix(opencode): avoid symlink-loop recursion during skill discovery

[coffeepenbit]

Issue for this PR

Fixes #16188

Type of change

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

What does this PR do?

Skill discovery could recurse indefinitely through symlink loops in external skill directories, which could stall startup and drive runaway memory use. This PR makes the scan cycle-safe by deduplicating visited directories by real path while still allowing symlinked skill roots such as .agents/skills. The fix still allows for skills via symlink.

This may also resolve some startup blank-screen or runaway-RAM reports. I think these also may have been related. I ran into these issues on M4 Pro macOS 15.7.4, causing my laptop to "hard" crash.

• opencode taking more and more ram until the whole computer crashes #14171
• OpenCode don't start after the migration #14636

How did you verify your code works?

• Ran bun test test/skill/skill.test.ts in packages/opencode
• Added a regression test that hangs on the old implementation and passes with this fix
• Verified the bug is present without the fix: the new regression test times out/hangs on the unfixed code under a guarded timeout
• Ran a local bun run dev startup check and no longer saw the runaway skill scan behavior

Screenshots / recordings

N/A

Checklist

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

[rekram1-node]

@atharvau STOP AI SPAMMING PRs it's incredibly annoying and it helps absolutely no one.

[rekram1-node]

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