feat(gateway): add mutable workspace checkpoints (#319)

[AntoineToussaint]

Closes #319.

Summary

• Lets Mind preserve and recover the strongest dirty workspace candidate without receiving project bytes or constructing Git or filesystem operations.
• Keeps checkpoint identity, storage admission, lease binding, and version compare-and-swap inside Codefly across run-control segments.
• Returns durable idempotency receipts so preservation, restore, and cleanup remain journalable across retries.

Risk

Restore replaces the mutable project tree with a verified staged candidate. It preserves VCS metadata and tracked generated content while intentionally discarding excluded plugin and runtime caches.

Test plan

• go test ./... -count=1
• go test ./code -count=1
• go test -race ./code -run ^TestWorkspaceCheckpointAgentIntegration$ -count=1
• go test ./... -run ^$ -count=1
• buf lint proto --path proto/codefly/base/v0/workspace_checkpoint.proto
• codefly generate proto --proto ./proto --output ./generated --local
• git diff --check