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

[indykish]

Summary

• Restores root-level SKILL.md and TRIGGER.md that were removed (see #139c5a8, "Delete github-pr-reviewer directory"), which left this repo without the fleet-bundle shape the agentsfleet fleet library onboarding flow expects.
• Content mirrors the agentsfleet monorepo's tests/fixtures/fleetbundle/github-pr-reviewer/{SKILL.md,TRIGGER.md} fixture, which the fleet-library test suite already treats as the reference shape for this bundle.
• Unblocks clean library onboarding and the published quickstart, tracked as an external-bundle blocker in M123_001_P0_API_CLI_DOCS_V2_LAUNCH_DOCUMENTATION.md.

Test plan

• Confirm agentsfleet fleet install github-pr-reviewer (or equivalent library onboarding path) resolves this repo and loads the bundle correctly
• gitleaks detect — no leaks found

Greptile Summary

This PR restores the root fleet bundle used to onboard the GitHub pull-request reviewer. The main changes are:

• Adds SKILL.md with the review workflow and operating constraints.
• Adds TRIGGER.md with the GitHub webhook and runtime declarations.
• Limits network access to the GitHub API host.

Confidence Score: 5/5

This looks safe to merge.

• No blocking issues found in the changed code.
• The two manifest names match and the runtime fields support the documented GitHub review flow.

Important Files Changed

Filename	Overview
SKILL.md	Defines the fleet identity, pull-request review steps, and comment-only constraints.
TRIGGER.md	Defines the matching GitHub webhook, HTTP tool, credential, network allowlist, and daily budget.

_{Reviews (1): Last reviewed commit: "fix: restore root SKILL.md and TRIGGER.m..." | Re-trigger Greptile}