Stage 1 complete: shared packages with full test coverage

- packages/schema: 15 Vitest tests (6 valid + 6 invalid frontmatter + 3 round-trip)
- packages/sanitize: fail-closed remark plugin + 12 private fixtures + 6 clean fixtures, 20 tests
- packages/observability: Pino + correlation IDs + redaction; 5 tests with 100-log validation
- packages/linkedin-client: Posts API client + token store; 10 tests; AES-256-GCM substituted for libsodium crypto_secretbox (Bun ESM bug, see docs/deferred-gates.md D-001)

50/50 tests pass across 4 packages. All Stage 1 DoDs verified.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

test/corpus/clean/clean_with_embeds.md

@@ -0,0 +1,12 @@
+---
+vault_path: Stargue/Projects/Publishing Engine/Posts/Embeds.md
+outlet: stargue.com
+embed_strategy: resolve
+---
+# Embeds
+
+Here is an embedded image:
+
+![[stargue-logo.svg]]
+
+And a follow-up paragraph.