docs(site): expand execution-context section in front-matter reference

[github-actions]

Summary

• The execution-context: section in front-matter.mdx described the plugin as one that "stages PR diff context" — making it sound PR-only. In reality it covers eight contributors across all trigger types.

Changes

• site/src/content/docs/reference/front-matter.mdx — rewrote the Execution Context section:
  • Fixed the description: "trigger-specific context files" instead of "PR diff context"
  • Added a contributor quick-reference table showing default/opt-in status and what each contributor stages
  • Replaced the minimal pr-only example with a full annotated YAML block covering all eight contributors (pr, workitem, pr.checks, manual, pipeline, ci-push, schedule, repo)
  • Clarified that auto-on contributors activate based on trigger config and opt-in contributors are off by default

Accuracy checks

• All contributor defaults verified against src/compile/types.rs (ExecutionContextConfig) and src/compile/extensions/exec_context/ contributor files
• Contributor table cross-checked against site/src/content/docs/reference/execution-context.mdx Fields section
• workitem default ("on when pr contributor is active") verified in WorkitemContextConfig doc comment

Validation

• cd site && npm run build:astro — 38 pages built, all internal links valid

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