ci(ff): support feature freeze automation

[v1v]

What is the problem this PR solves?

This PR migrates the fleet-server release automation from Makefile-based scripts to a pure Go implementation using Mage, eliminating all external tool dependencies.

✨ Key Features

• Mage Commands for complete release workflow automation
• Pure Go Implementation - no external CLI tools (hub, gh, sed, yq, Python)
• DRY_RUN Mode - safe testing without pushing changes or creating branches
• Comprehensive Testing - 60%+ test coverage with unit tests
• Detailed Documentation - both quick reference and comprehensive guides

How does this PR solve the problem?

Move away from centralised approach but support the FF automation as part of #6343

How to test this PR locally

export PROJECT_OWNER="your-user"
export CURRENT_RELEASE="9.5.0-test"
export GITHUB_TOKEN=$(gh auth token)
export DRY_RUN=true

# Dry run first
mage release:runMajorMinor

# Review changes
git diff

# Run for real on your fork
export DRY_RUN=false
mage release:runMajorMinor

Design Checklist

• I have ensured my design is stateless and will work when multiple fleet-server instances are behind a load balancer.
• I have or intend to scale test my changes, ensuring it will work reliably with 100K+ agents connected.
• I have included fail safe mechanisms to limit the load on fleet-server: rate limiting, circuit breakers, caching, load shedding, etc.

Checklist

• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in ./changelog/fragments using the changelog tool

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