feat(sync): write local prompt variation files

[ctawiah]

Context

Bootstrap and sync need one local store for reading and writing .launchdarkly prompt variation files safely.

Summary

• Renders agent and completion variations in the format accepted by the local parser.
• Creates workspaces and adds selected variations without overwriting existing files.
• Rolls back partial multi-file writes.
• Loads project and variation metadata used by planning and direct apply.
• Validates path segments, modes, and supported message roles.

Verification

• go test ./internal/sync/local