Fix smoke workflow and JSON-LD build

[bdelanghe]

Summary

• set SITE_URL for smoke container and make container logging resilient
• gate smoke/publish/deploy jobs and skip deploy on non-main branches
• fix JSON-LD site build pipeline (loader registration, layout, empty vault handling, URL normalization)
• harden AI exporters metadata extraction and fix test path

Testing

• docker compose run --rm unfold-test deno test -A src/unfold/pipeline/validate_test.ts
• docker compose run --rm unfold-test sh -c 'set -e; mkdir -p example-vault/.obsidian; cat > example-vault/hello.md <<EOF

---

title: Hello

Hello

Test content.
EOF
SITE_URL=http://localhost:3000 SITE_OUTPUT_DIR=.unfold/site VAULT_PATH=example-vault deno test -A src/unfold/exporters/exporters_ai_test.ts; rm -f example-vault/hello.md; rm -rf example-vault/.obsidian'