docs: Include Agent Skill Markdown in linting

[aoirint]

Warning

This issue was created with assistance from LLMs.

Summary

• Stop excluding repository-local Agent Skill Markdown from Markdown linting and format it to pass the shared rules.

Details

• Follow-up from ci: enforce Markdown lint and clarify quality checks #39, where .agents/** was excluded so Markdown linting could be introduced without expanding the
  first PR into a large Agent Skill formatting change.
• This is related to Document build CI and composite action intent #40 because both issues came from preserving follow-up work discovered during the lint CI rollout.
• .markdownlint-cli2.yaml currently ignores .agents/** so the new Markdown lint CI can be introduced without
  immediately reformatting Agent Skill files and worktree-local notes.
• Repository-local Agent Skills under .agents/skills/ should eventually follow the same Markdown quality policy as
  the rest of the repository.
• The work should avoid linting transient implementation worktrees under .agents/worktrees/ unless there is a clear
  reason to include them.

Acceptance Criteria

• Agent Skill Markdown under .agents/skills/ is included in Markdown linting.
• Existing Agent Skill Markdown is reformatted to pass markdownlint-cli2.
• Transient worktree content remains excluded or is handled by a documented rule.
• CI and local Markdown lint commands pass after the exclusion is narrowed or removed.