feat: improve pr-review skill with no-ghost-review and multi-OS rules

[eskenazit]

Related Issue

Closes #413

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What does this PR do?

Improves the pr-review skill to prevent two mistakes observed in practice during the PR #395 review:

1. Ghost reviews from individual thread replies
Posting replies via pulls/comments/{id}/replies individually before the main review creates a separate ghost review object per reply on GitHub. The skill now explicitly forbids this pattern and documents how to bundle thread replies inside a single POST /reviews call using the in_reply_to field.

2. Multi-OS coverage for the in_reply_to example
Both PowerShell (Windows) and bash (Linux/macOS) variants are provided, consistent with the rest of the skill.

Version bumped: 1.1.0 -> 1.2.0.

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Tests

No automated tests -- this is a documentation/skill file change. The ghost-review pattern was observed in practice during PR #395 and the fix was validated against the GitHub API.

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Checklist

• CI is green (build, tests, schema validation, security scans)
• Rebased on latest main
• Small and focused -- one concern per PR
• Commit messages follow Conventional Commits

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Agent Context (optional)

agent_name: GitHub Copilot
llm: Claude Sonnet 4.6
tool: VS Code Chat
confidence: high
source_event: ghost reviews observed during PR #395 review session
discovery_method: runtime_observation
review_focus: .agents/skills/pr-review/SKILL.md

[jeremnaf]

LGTM