feat(manifest): add JSON schema for editor autocompletion

[zimeg]

Summary

• Adds a $schema property to manifest.json pointing to slackapi/manifest-schema
• Enables typeahead autocompletion and validation in editors that support JSON Schema (e.g., VS Code, vim with LSP)

Test plan

• Open manifest.json in an editor with JSON Schema support (e.g., vim manifest.json with an LSP configured, or VS Code)
• Confirm typeahead / autocompletion suggestions load for manifest properties
• Confirm schema validation highlights any invalid fields

References

• slackapi/manifest-schema

🤖 Generated with Claude Code

[WilliamBergamin]

Yup+ 🔢

[zimeg]

@WilliamBergamin Thank you for the kind review once again 🐢 💨