Set gamma as default branch

[Copilot]

Updates repository configuration and documentation to establish gamma (v2.0) as the default branch, replacing main (v1.0).

Changes

• CI configuration: Reordered branch triggers in .github/workflows/ci.yml to prioritize gamma in push/PR events
• Documentation structure: Updated branch ordering and decision trees across README, branch guides, and version comparison docs
• Branch semantics: Relabeled gamma from "alpha/cutting-edge" to "default/primary development", main from "stable" to "legacy stable"
• Navigation updates: Reordered quick-start commands to present gamma first with ⭐ marker

Documentation Updates

File	Key Change
README.md	Removed "warning" tone, positioned gamma as primary
.git-branches-guide.md	Reordered checkout commands (gamma → main → beta)
docs/BRANCH_VERSION_MAP.md	Updated header with "DEFAULT BRANCH: GAMMA"
docs/VERSION_COMPARISON.md	Inverted decision tree to start with gamma

No functional code changes. Branch permissions and GitHub repository settings require separate admin action.

Original prompt

set gamma as the default branch

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