feat: integrate schema.lua runtime validation

[mcanouil]

Summary

• Add the shared schema.lua module to _modules/.
• Rename the existing gitlink-specific schema module from schema.lua to platform-schema.lua to avoid naming conflicts.
• Update platforms.lua to require platform-schema instead of schema.
• Wire up schema.validate_options() in gitlink.lua to validate extension options against _schema.yml at runtime (Tier 2: error reporting only).

Test plan

• Run quarto render example.qmd and confirm no runtime errors.
• Verify platform validation still works (platform-schema.lua).
• Verify that valid options produce no warnings.
• Test with intentionally invalid options and verify error/warning messages are logged.