Add governance guide for gh aw env defaults and scope inheritance

[Copilot]

This PR adds dedicated governance documentation for environment defaults in gh-aw, focused on how to configure GH_AW_DEFAULT_* values with gh aw env and how scope inheritance works across enterprise, organization, and repository levels. It closes the gap between variable reference docs and operational rollout guidance for multi-repo governance.

• New guide: environment governance

  • Added docs/src/content/docs/guides/governance.md.
  • Documents:
    • gh aw env get / gh aw env update workflows
    • default_* YAML schema and deletion semantics (null)
    • rollout model (enterprise baseline → org policy → repo exceptions)
    • precedence/percolation behavior and frontmatter override rules
    • compile-time vs runtime default resolution

• Navigation updates

  • Added Governance under Guides in docs/astro.config.mjs for discoverability.

• Cross-reference improvements

  • Linked the new guide from reference/environment-variables.md in Related Documentation.

• Representative snippet

  default_max_ai_credits: "5M"
  default_max_daily_ai_credits: "15M"
  default_max_turns: "12"
  default_timeout_minutes: "30"
  default_model_copilot: "gpt-5-mini"
  default_model_claude: "claude-haiku-4-5"
  default_model_codex: "gpt-5.4-mini"
  default_detection_model: "gpt-5.5-mini"
  default_utc: "-08:00" # UTC offset for rendered CLI timestamps

[Copilot]

Pull request overview

Adds a governance-focused guide to help operators roll out gh aw env (GH_AW_DEFAULT_*) defaults across enterprise/org/repo scopes, and wires it into the docs navigation and related references.

Changes:

• Added a new guide documenting gh aw env get/update, scope inheritance, precedence, and compile-time vs runtime default resolution.
• Added “Governance” to the Guides sidebar for discoverability.
• Linked the new guide from the environment variables reference page.

Show a summary per file

File	Description
docs/src/content/docs/guides/governance.md	New governance guide describing defaults management, precedence, and rollout patterns.
docs/src/content/docs/reference/environment-variables.md	Adds cross-link to the governance guide in Related Documentation.
docs/astro.config.mjs	Adds “Governance” entry under Guides in the sidebar.

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• Files reviewed: 3/3 changed files
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• Review effort level: Low