feat(agent-development): add cross-references to related skills

[sjnims]

Summary

The agent-development skill includes an excellent comparison table (agents vs commands vs skills) but doesn't link to the related skills that could help users learn more about each component type.

Current State

Lines 19-47 in SKILL.md contain:

• Comparison table (agents vs commands vs skills)
• "Choose Agents When" section
• "Choose Commands When" section
• "Choose Skills When" section

These sections describe when to use each component but don't cross-reference the development skills for those components.

Proposed Changes

Add cross-references to related skills:

1. In the comparison table or after it, add a note like:

   > **See also:** For command development, load the `command-development` skill. For skill development, load the `skill-development` skill.

2. In "Choose Commands When" section, add:

   For command development guidance, see the `command-development` skill.

3. In "Choose Skills When" section, add:

   For skill development guidance, see the `skill-development` skill.

Rationale

Users reading the agent-development skill often need to understand all three component types. Cross-references improve navigation and help users discover related skills they may need.

Acceptance Criteria

• Cross-references added to comparison section
• Links point to actual skill names users can invoke
• Word count remains in ideal range (currently 1,623 words)

Related

• Identified during comprehensive skill review against Claude Code plugin best practices

[sjnims]

PR submitted: #21