feat(sync): write local prompt variation files

[ctawiah]

Context

The sync flow also needs a safe way to materialize LaunchDarkly prompt variations in a local .launchdarkly workspace. This storage layer is shared by first-time interactive bootstrap and future server-to-local operations, while keeping file generation separate from API selection and terminal UI concerns.

The main safety requirement is that existing local work is never overwritten. New files are rendered in the same format accepted by the local parser, and multi-file writes are rolled back if any file cannot be created. This PR does not fetch or select remote variations.

Summary

• 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.
• Exposes rendering separately so dry runs can show exact file paths and content without writing.
• 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