docs: add SKILL_STYLE.md and align contribution guidelines

[h30s]

Closes #3

What changed

• Added SKILL_STYLE.md at the repo root defining the OxCode skill philosophy, the six core disciplinary rules with real worked before/after examples, frontmatter reference, guidelines for choosing tools and workspace, and testing standards.

• Updated CONTRIBUTING.md to link SKILL_STYLE.md, state that PRs ignoring it will be sent back, and require before/after proof on real tasks.

• Updated README.md to link SKILL_STYLE.md, fix plugin paths in the table, and list triage.

• Updated template/SKILL.md so new skills scaffold directly from the style guide (defaulting to tools: chat for judgement skills and outlining disciplined rule sections).

Before and After Comparison on a Real Task

• Task: Reviewing a diff where a method signature changed in user_service.py
  without all callers included in the changeset.

• Before (generic/unconstrained prompt):
  "Ensure all caller functions across the codebase handle null values returned by
  this updated method." (guesses past missing context and makes claims without checking).

• After (following SKILL_STYLE.md rules):
  "Cannot verify caller handling from this diff because user_service.py is outside
  the changeset." (refuses to claim what wasn't checked; treats missing context as a finding).

What was deliberately left out and why

• Model-specific prompt tuning or mentioning LLM providers: OxCode routes dynamically via capability, so naming models or prompt tricks tied to a specific provider version was excluded to avoid obsolescence.

• Prescriptive domain templates: The guide defines the discipline and standard of
  truth rather than attempting to catalog every possible software domain.

Tool Configuration Verification

• Read-only review and judgement skills default to tools: chat.

• Active skills (like test-hardening) require tools: full to execute mutations and run tests on disk rather than hallucinating outputs.