[instructions] Sync instruction files with release v0.87.2

[github-actions]

Summary

Synchronizes the .github/aw/*.md instruction files with upstream release v0.87.2. The update documents a new experimental plugins: (Agent Plugins) field, a hide-comment.discussions permission field, and automatic model-fallback disabling when custom provider endpoints are configured. No source code is changed; this is a documentation-only sync.

Change Classification

• Type: docs
• Scope: .github/aw instruction files (engine configuration, safe-outputs, syntax reference)
• Breaking: No

Key Changes

File	Change	Impact
.github/aw/configure-agentic-engine.md	Documents new experimental behaviors.plugins field that opts a custom engine into top-level plugins: support	Low
.github/aw/safe-outputs-automation.md	Documents optional discussions: true opt-in for discussions:write permission when hiding discussion comments	Low
.github/aw/safe-outputs-runtime.md	Adds hide-comment.discussions to the list of fields influencing permission computation	Low
.github/aw/syntax-agentic.md	Clarifies that model-fallback is auto-disabled when engine.env sets OPENAI_BASE_URL/ANTHROPIC_BASE_URL	Low
.github/aw/syntax-tools-imports.md	Adds a full "Agent Plugins (plugins:)" section: syntax, ref resolution rules, per-engine support, and import merge behavior	Low
.github/aw/syntax.md	Adds an index reference link to the new Agent Plugins documentation	Low

Impact Assessment

• No downstream code, compiler, or runtime behavior changes — this PR only updates instructional documentation to reflect features already present upstream (experimental plugins: field, hide-comment.discussions, automatic model-fallback disabling).
• Keeps in-repo instruction files (used as ambient context by agentic workflows/Copilot) consistent with the v0.87.2 release notes, reducing risk of stale guidance being surfaced to agents or contributors.

Commits

919b459fb [instructions] Sync instruction files with release v0.87.2
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<!-- gh-aw-agentic-workflow: PR Description Updater, engine: copilot, model: auto, id: 32472146158, workflow_id: pr-description-caveman, run: https://github.com/github/gh-aw/actions/runs/32472146158 -->

[github-actions]

✅ Looks good — thanks for the documentation sync! This PR aligns perfectly with the project's agentic development workflow and contribution guidelines:

• ✓ Follows the established process — generated by the Instructions Janitor automation, which is the approved mechanism for syncing docs with releases.
• ✓ Focused change — updates six instruction files to document three behavioral changes from v0.87.2.
• ✓ Complete description — clearly explains what was audited and what was fixed.
• ✓ No new dependencies or unrelated changes.

This is ready for review by a maintainer.

Generated by ✅ Contribution Check · auto · 45.8 AIC · ⌖ 6.18 AIC · ⊞ 9.2K · ◷

[github-actions]

🎉 This pull request is included in a new release.

Release: v0.87.4