feat: add dynamic render config

[karanbalani]

Summary

• Add target-repo .shiplog/render.json support with a bundled fallback render config.
• Render Markdown from JSON blocks and plain read-only SQL queries, then append the required shiplog footer.
• Render and publish each configured target independently, with GitHub Actions matrix isolation and collapsible README previews in the action summary.

Notes

• Render SQL no longer has built-in Shiplog parameters; users write normal Postgres SQL against their Shiplog database tables.
• Explicit content and inputPath publish flows still work for fixed-content publishing.

Validation

• bun test 165 pass
• bun run typecheck
• bun run lint
• bun run format:check