Partial clone and sparse checkout opt-in

[BramVR]

Parent

#70

What to build

Add an opt-in Git-native lazy hydration slice using partial clone and sparse checkout through Clone Strategy. This should be explicit, testable, and separate from any future VFS or hydration stubs.

Acceptance criteria

• A project or command can request partial clone and sparse checkout explicitly.
• Full clone remains the default behavior.
• Agent Run Contract records partial/sparse strategy details when used.
• Tests use local Git fixtures where possible and skip only unsupported Git capabilities with clear diagnostics.
• Docs state this is Git-native laziness, not VFS behavior.

Blocked by

• Clone Strategy full-clone baseline #84