Write architecture doc + 3 ADRs

[NirmalKBandara]

Goal

Document the architecture and major engineering decisions.

Acceptance Criteria

• docs/architecture.md completed
• Service architecture explained
• OpenChoreo resource chain explained
• Environment/deployment model explained
• Service dependencies explained
• ADR: monorepo decision
• ADR: SQLite-per-service decision
• ADR: Docker over Buildpacks decision