feat: add proto-first guide as databricks-apps reference

[keugenek]

Summary

Adds proto-first design guidance as reference docs inside the existing databricks-apps skill (per review feedback — not a standalone skill).

Files added:

• skills/databricks-apps/references/appkit/proto-first.md — 4-phase workflow: decompose → contracts → generate → implement
• skills/databricks-apps/references/appkit/proto-contracts.md — concrete proto↔plugin mappings for files, lakebase, jobs

SKILL.md updated: Added "Typed data contracts" row to the Required Reading table.

What it covers

• Module decomposition (files, lakebase, jobs, analytics, server boundaries)
• Proto style rules (one file per boundary, ≤12 fields, snake_case)
• Proto→Lakebase DDL type mapping
• Proto→Plugin method mapping (files upload, lakebase insert, job params)
• Volume layout convention from proto messages
• Common traps (any at boundary, JSON.parse, god messages)

Documentation safety checklist

• Examples use least-privilege permissions
• Sensitive values are obfuscated
• No insecure patterns introduced

[arsenyinfo]

From my understanding of current granularity, that should not be a separate skill in existing taxonomy.
Even appkit itself is not a separate skill but a core recommendation in databricks-apps skill. This proto-first thing is an implementation best practice, i'd expect it to be part of apps skill referenced close to other core appkit practices.

[keugenek]

From my understanding of current granularity, that should not be a separate skill in existing taxonomy. Even appkit itself is not a separate skill but a core recommendation in databricks-apps skill. This proto-first thing is an implementation best practice, i'd expect it to be part of apps skill referenced close to other core appkit practices.

agree it is more like a meta-skill - how would you recommend implementing it then? (compared to current approach)

[keugenek]

/merge

[keugenek]

isaac-bot pull master and resolve any conflicts.