Add Agda certifier integration with source location tracking

[zeme-wana]

• Refactor source location tracking in plutus-tx-plugin to flow through ReaderT
  CompileContext (ccCurrentLoc) instead of invasive function parameters on
  compileExpr.
• Add --certify[=DIR] plugin option that invokes the Agda certifier for each
  compiled Plutus script, embedding package name, module name, and source location in
  the certificate metadata.
• Simplify compileMarkedExprs (remove stateful go lastLoc loop), stripAnchors
  (single pattern match instead of recursive go), and package name resolution
  (lookupUnit / thisPackageName / stripVersionFromUnitId fallback chain).
• Improve Certifier.hs: include certifier report in error messages, use
  createDirectoryIfMissing, remove noisy console output.
• Encode TH source locations in structured format (encodeTHLoc) so the plugin can
  recover RealSrcSpan from the type-level string.
• Add CI workflow (plinth-certifier-tests.yml) and test script
  (scripts/plinth-certifier-tests.py).