Update SKILL.md instructions for branch naming and git ops

[skarim]

Updates to the SKILL.md file used in npx skills to clarify behavior for agents

• Clearer prefix instructions: Added "Branch naming" subsection recommending prefixes as default workflow
• Clarified staging changes across branches: Expanded "Staging changes deliberately" section to show the primary use case — splitting changes across branches with git add + gh stack add, not just multiple commits on one branch.
• Simplified squash-merge recovery docs: Removed git rebase --onto implementation details since agents only need to know that gh stack rebase|sync handles squash-merged PRs automatically

[Copilot]

Pull request overview

Updates the gh-stack skill documentation to standardize branch naming around prefixes (e.g., feat/...) and to clarify how gh stack add should be used when a prefix is configured.

Changes:

• Updates stack diagrams/examples to use prefixed branch names (feat/...).
• Adds guidance that when a prefix is set, gh stack add should be given only the suffix.
• Expands branch naming + staging workflow explanations and updates the quick-reference commands.

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