feat: add on-demand skill loading for agents

[ericlake]

Implement the Agent Skills specification for Aura. Skills are directories containing a SKILL.md file with YAML frontmatter. The LLM sees a catalog of available skills in its system prompt and calls a load_skill tool to retrieve detailed instructions only when needed, keeping the base prompt small.

• Add SkillsConfig and LocalSkillSource to aura-config with spec-compliant name validation (1-64 chars, lowercase alphanumeric
  • hyphens, must match directory name)
• Add skill discovery scanning source directories for SKILL.md files with AURA_SKILLS_DIR env var fallback
• Add LoadSkillTool implementing rig::tool::Tool for on-demand loading of skill instructions and reference files
• Inject skill catalog into agent system prompt during build
• Thread config_dir through web server to resolve relative skill paths
• Add /app/skills directory and AURA_SKILLS_DIR env var to Dockerfile
• Sort reference files by name for deterministic output
• Warn (without truncating) when skill content exceeds 512KB
• Add unit tests for name validation, frontmatter parsing, and skill discovery (dedup, name mismatch, relative paths)

Ref: LOG-23254

[github-actions]

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[ericlake]

I have read the CLA Document and I hereby sign the CLA

[github-actions]

Test results for Rust

335 tests  +30   335 ✅ +30   6s ⏱️ -1s
  4 suites ± 0     0 💤 ± 0 
  1 files   ± 0     0 ❌ ± 0 

Results for commit 00070b7. ± Comparison against base commit 73b3442.