feat: add native Agent Skills support to ds4-agent

[fry69]

Here is an implementation to teach ds4-agent reading skills on startup.

This PR soft depends on #379 to keep the current working directory available during startup.

Without keeping the working directory stable skill lookup will only find what is in the home directory, not in the project directory.

Automatically generated PR message follows:

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Summary

Add native Agent Skills support to ds4-agent, following the open Agent Skills
specification (agentskills.io). 354 lines added, all in ds4_agent.c — no other
files touched.

Changes

New DSML tool: activate_skill

• The model can call activate_skill(name) to load a skill's SKILL.md.
• The tool looks up the skill by name in the discovered list, reads the file,
  and returns the content wrapped in <skill_content> tags with the skill
  directory path for relative-path resolution.
• Schema and behavioural instructions are in the system prompt.

Skill discovery

• Scans <cwd>/.agents/skills/ and ~/.agents/skills/ at session start.
• Parses YAML frontmatter (name, description) with lenient error handling.
• Skills with empty descriptions are skipped; malformed YAML is warned about.

Skill catalog

• After the system prompt is loaded, a <available_skills> XML catalog is
  appended as a separate user message. This keeps the system prompt static
  and KV-cacheable while the catalog can change independently.
• The catalog includes behavioural instructions telling the model to call
  activate_skill when a task matches a skill's description.

Slash commands

• /skills — list discovered skills with names and descriptions.
• /refresh — rescan skills directories, compare catalog text, and if changed,
  append the updated catalog at the end of the transcript. No full prefill is
  needed — only the new tokens are processed. The model sees the most recent
  catalog and uses it going forward.

Startup notification

• Prints "Skills: N loaded" or "Skills: none found" at session start.

Backward compatibility

No breaking changes. If no skills directories exist, the agent behaves exactly
as before — no catalog message is appended, and /skills reports zero skills.