Set default branch to main for clone creation (#662)

Summary

• When creating a clone without specifying a branch or snapshot, default to "main" branch
• When a specific snapshot is requested but no branch specified, use the snapshot's branch (maintains consistency)
• UI: Prioritize "main" branch in the branch selector dropdown

Changes

API (engine/internal/cloning/base.go):

• Default to "main" only when neither branch nor snapshot are specified
• Use snapshot's branch when snapshot is specified but branch is not

UI (ui/packages/shared/pages/CreateClone/index.tsx):

• Default branch selector to "main" if available, fallback to first branch

Tests (engine/internal/cloning/base_test.go):

• Added TestDefaultBranchForCloneCreation covering all branch selection scenarios

Behavior

Branch Specified	Snapshot Specified	Result
No	No	Default to "main"
No	Yes	Use snapshot's branch
Yes	Any	Use specified branch

Fixes: #662 (closed)