Restore root SKILL.md and TRIGGER.md for fleet library onboarding

SKILL.md

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+---
+name: github-pr-reviewer
+description: Reviews GitHub pull requests and posts review comments.
+version: 0.1.0
+---
+# GitHub Pull Request reviewer
+
+Reviews open pull requests and leaves focused, constructive review comments.
+
+## Goal
+For each pull request that wakes this fleet, read the diff and post review
+comments that flag correctness bugs, missing tests, and risky changes.
+
+## Steps
+1. Read the pull request diff.
+2. Identify correctness, security, and test-coverage gaps.
+3. Post one review comment per finding with `http_request`.
+
+## Constraints
+- Comment only — never push, merge, approve, or close.
+- Stay within the declared GitHub network host.

TRIGGER.md

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+---
+name: github-pr-reviewer
+x-agentsfleet:
+  triggers:
+    - type: webhook
+      source: github
+      events:
+        - pull_request
+  tools:
+    - http_request
+  credentials:
+    - github
+  network:
+    allow:
+      - api.github.com
+  budget:
+    daily_dollars: 2.0
+---
+# Wake rule
+
+Wakes on GitHub `pull_request` webhook events for the connected repository.