fix: add missing compound-docs agent persona

[terry-li-hm]

Summary

The /workflows:compound command fails with "Agent type not found" because it tries to spawn a compound-docs agent, but only the skill exists—no agent persona file.

This PR adds the missing agents/workflow/compound-docs.md with:

• Proper YAML frontmatter (name, description, model: haiku)
• Agent persona describing documentation responsibilities
• Quality standards and workflow steps
• Output format matching the skill expectations

Test plan

• Run /workflows:compound after solving a problem
• Verify agent spawns successfully
• Verify documentation is created in docs/solutions/

Fixes #138

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🤖 Generated with Claude Code

[terry-li-hm]

Closing in favour of #156, which takes the opposite (and I think better) approach: these components shouldn't have agent files at all.

Agent personas are eager-loaded into every session's Task tool description (~100-200 tokens each, every API call). Skills are lazy-loaded (name + one-liner). For components that work fine as skills, adding agent personas adds permanent token overhead for all users with minimal benefit.

The underlying issue — /workflows:compound failing because it tries to spawn compound-docs as an agent — should be fixed in the workflow command itself (invoke via Skill tool instead of Task agent spawn), rather than by adding an agent persona that taxes every session.

See #156 for the broader proposal.