feat: add disabled_skills config and --no-skills CLI flag

[spboyer]

Summary

Implements #126 — allows users to control which skills are loaded during evals, including a "none" option to disable all skills.

Changes

New config field: disabled_skills

• ["*"] disables all skill loading entirely
• Specific skill names (e.g., ["noisy-skill"]) exclude individual skill directories
• Works by matching against both full path and basename of skill_directories entries

New CLI flag: --no-skills

• Equivalent to disabled_skills: ["*"] in YAML
• Overrides any skill_directories configured in the eval spec

Implementation details:

• Config.AllSkillsDisabled() — checks for wildcard disable or skill_directories: ["none"]
• Config.FilteredSkillPaths() — returns skill paths with disabled entries removed
• ExecutionRequest.NoSkills — signals CopilotEngine to skip getSkillDirs() and buildSkillSystemMessage()
• validateRequiredSkills() — early returns when all skills disabled
• Updated schema, CLI reference, eval-yaml guide, and schema reference docs

Testing

14 new test cases across 3 test functions:

• TestConfig_AllSkillsDisabled (6 subtests)
• TestConfig_FilteredSkillPaths (6 subtests)
• TestBenchmarkSpec_DisabledSkillsDeserialization (2 subtests)

All existing tests pass. Docs site builds cleanly.

Usage

# Disable all skills
config:
  disabled_skills: ["*"]

# Disable specific skills
config:
  skill_directories: [./skills/a, ./skills/b, ./skills/c]
  disabled_skills: [b]  # only a and c are loaded

# CLI flag
waza run eval.yaml --no-skills

Working as Linus (Backend Developer)

Closes #126