feat(sync): CPLYTM-1290 add diagram block for Kroki rendering

[sonupreetam]

Summary

• Adds rewriteDiagramBlocks transform to transform.go that converts standard diagram code blocks (```mermaid, ```plantuml, ```d2, etc.) to the ```kroki {type=...} format Hugo's Kroki render hook expects
• Wires the transform into all three sync pipelines: syncConfigSource (opt-in via rewrite_diagrams: true), syncRepoDocPages (always-on), and processRepo README processing (always-on)
• Adds RewriteDiagrams field to the Transform config struct for per-file control in sync-config.yaml
• 12 unit tests covering all supported diagram types, edge cases (dot→graphviz normalisation, already-kroki blocks, inline references, trailing whitespace)

Test plan

• All 12 new TestRewriteDiagramBlocks subtests pass
• Full test suite passes (all existing tests unaffected)
• Code compiles cleanly, no lint errors
• Verify end-to-end: sync a repo containing a ```mermaid block and confirm Hugo renders it as SVG via Kroki
• - Dependency: feat(cli): CPLYTM-1291 content sync tool org wide doc #2