* feat(artifact): add MD+frontmatter export/import core for playbooks & conventions
Pure-Go internal/artifact package: Encode/Decode a single playbook or
convention item to a portable Markdown + YAML-frontmatter artifact with
deterministic key order and a provenance block. Field-key tables map item
fields to frontmatter per kind; FieldKeysForKind is exported for the
server layer. Round-trip + golden + determinism + error-case tests.
Server-side validation, forgiving coercion, slug-collision handling, and
YAML input limits are deferred to Phase 2 (HTTP boundary).
Implements Phase 1 of PLAN-1867 (TASK-1868, TASK-1869, TASK-1870).
Claude-Session: https://claude.ai/code/session_01KmxkPxLksjf1pmrZDpsnTJ
* fix(artifact): tolerate CRLF line endings on decode
Normalize CRLF to LF at Decode's entry point so artifacts authored or
transported on Windows decode identically (fence detection + body
preservation operate on canonical LF). Addresses Codex P2 round 1.
Claude-Session: https://claude.ai/code/session_01KmxkPxLksjf1pmrZDpsnTJ
xarmian