docs: Branch Protection Rules & Mandates

[Tristan578]

This PR updates the repository documentation to reflect the new GitHub branch protection rules:

• Mandatory PRs for \main\
• 75% coverage hard gate
• Linear history (Squash/Rebase)
• Conversation resolution required
• .gemini/GEMINI.md\ added as foundational mandate for agents.

[Copilot]

Pull request overview

Updates repository documentation to reflect newly enforced GitHub branch protection and engineering mandates, plus introduces a foundational agent mandate document under .gemini/.

Changes:

• Update README key enhancements to mention hard-enforced CI/security/linear-history standards and the new GitHub metadata storage approach.
• Revise CONTRIBUTING PR workflow to match protected main, CI gates (coverage/security), required resolved conversations, and linear history.
• Add .gemini/GEMINI.md defining agent engineering standards and PR workflow mandates.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
README.md	Documents hard enforcement standards and updates how GitHub Issue metadata is stored.
CONTRIBUTING.md	Aligns contributor PR process with protected-branch/CI/merge requirements.
.gemini/GEMINI.md	Adds authoritative agent mandates for testing/coverage and PR workflow rules.

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