docs(site): document exec-context-pr.js bundle in ado-script reference

[github-actions]

Summary

• Developers reading ado-script.mdx now see all three bundles (gate.js, import.js, exec-context-pr.js) instead of just two
• The new section explains what exec-context-pr.js does, how merge-base resolution works, the trust boundary, and its full env-var contract
• The workspace layout tree, release-workflow paragraph, and wiring table are updated to reflect the real workspace structure

Changes

• site/src/content/docs/reference/ado-script.mdx
  • Updated page description and intro to list all three bundles
  • Added ## What exec-context-pr.js does section with merge-base strategy, trust-boundary notes, and env-var contract table
  • Updated workspace layout tree to include exec-context-pr/ source directory and exec-context-pr.js build output
  • Updated release-workflow paragraph to mention exec-context-pr.js
  • Renamed ### Agent job wiring subsection to reflect dual consumers (import.js and exec-context-pr.js)
  • Expanded ### What gets emitted, by case table with an on.pr w/ exec-context column (8-row matrix)
  • Added note pointing to exec-context-pr.js as a real working example in ### Add a new bundle
  • Added Execution Context to ## See also

Accuracy checks

• Bundle existence and source layout confirmed against scripts/ado-script/src/exec-context-pr/
• Env-var contract verified against src/compile/extensions/exec_context/pr.rs::prepare_step() and scripts/ado-script/src/exec-context-pr/validate.ts
• Merge-base resolution paths confirmed against scripts/ado-script/src/exec-context-pr/merge-base.ts
• Activation condition (on.pr + execution-context.pr.enabled != false) verified against src/compile/extensions/exec_context/pr.rs::should_activate()
• Trust-boundary description verified against src/compile/extensions/exec_context/pr.rs module docstring and git.ts::bearerEnv
• Wiring ownership confirmed against src/compile/extensions/ado_script.rs::prepare_steps() (installs bundle when exec_context_pr_active)

Validation

• cd site && npm ci && npm run build — passed, all 34 internal links valid

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