feat(config): load commands from .agents

[Arcadi4]

Issue for this PR

Closes#27972

Type of change

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

What does this PR do?

Adds command discovery from .agents/commands/**/*.md, matching the existing .agents/skills convention.

Previously slash command markdown was discovered from .opencode/command and .opencode/commands only. This change scans project .agents directories and global ~/.agents, then reuses the existing command loader. The command-name derivation now recognizes .agents/commands, so nested commands keep the existing nested/name behavior.

Prior related work: #12486 requested the same .agents/commands support and was closed as stale, and #12488 was an unmerged duplicate implementation PR that was also closed as stale. #14240 is related but broader because it asks for configurable command and agent search paths.

How did you verify your code works?

Ran:

• bun test test/config/config.test.ts --timeout 30000 --test-name-pattern "loads commands"
• bun typecheck

Also ran a manual loader probe against a temporary .agents/commands tree containing qa.md and nested/check.md, and confirmed both commands loaded with the expected names and templates.

Screenshots / recordings

Not applicable; this is a config loading change.

Checklist

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

If you do not follow this template your PR will be automatically rejected.

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