feat(sync): write local prompt variation files

[ctawiah]

Summary

Adds the variation-only local store used by first-time bootstrap and later server-to-local pulls. This focused layer builds on the plan preview command.

• Writes agent and completion variations as .prompt.md files that round-trip through the local parser.
• Creates a complete .launchdarkly workspace atomically for first-time bootstrap.
• Adds selected variations without overwriting existing files and rolls back partial writes on failure.
• Validates path segments, variation modes, and supported message roles.
• Stays limited to Phase 1 prompt variations and adds no Git requirement or source metadata file.

Verification

• go test ./internal/sync/local -count=1
• Focused lint for ./internal/sync/local