feat(sync): load local prompt variations

[ctawiah]

Context

This is the foundation for Phase 1 of code canonical sync for AI Config prompt variations. The CLI needs a stable local format that people can edit and that later planning requests can send to Gonfalon without recreating or interpreting server-owned fields.

This PR is intentionally local-only. It defines how prompt files are discovered, parsed, validated, and converted into sync resources. It does not call Gonfalon, calculate fingerprints, create plans, or apply changes.

Summary

• Supports agent instructions and completion messages in an editor-friendly .prompt.md format.
• Uses a maintained frontmatter parser while preserving strict YAML fields and the prompt body exactly.
• Includes variation identity, name, model configuration, model values, and output format.
• Keeps the server-owned AI Config variation comment out of local files and sync payloads.
• Walks project directories and sorts variations by project and lookup key for stable request construction.
• Rejects malformed or unsupported files with errors that identify the failing path.
• Leaves canonicalization, fingerprinting, and comparison to Gonfalon.

Verification

• go test ./internal/sync/... -count=1

---

Note

Overview
Introduces Phase 1 local loading for AI Config prompt variations: the CLI can scan a .launchdarkly workspace and turn on-disk .prompt.md files into sorted SyncedResource values with JSON payloads, without talking to Gonfalon yet.

Adds the internal/sync domain model (SyncedResource, Variation, completion Messages, agent vs completion modes) and internal/sync/local.Compile, which walks .launchdarkly/<project>/configs/<config-key>/*.prompt.md, skips unrelated files, and fails with path-bearing ParseErrors when validation breaks.

File format: YAML front matter (formatVersion: 1, strict known fields via yaml decoder) carries identity, upsert, model config, and outputFormat; the markdown body becomes either trimmed agent instructions or completion messages parsed from <system>, <user>, and <assistant> tags (including nested tags), with untagged completion bodies treated as a single system message. Front matter cannot include instructions/messages; filename stem must match key.

Deps: github.com/adrg/frontmatter (and indirect TOML/yaml.v2 entries in the module graph). Covered by broad unit tests for compile, sort order, edge cases, and parse validation.

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