docs: expand architecture guide with collector patterns

[rokernel]

Summary

• Expand docs/architecture.md with code-backed collector design details and runtime patterns.
• Document both cache models used in this repo (scrape-time and background refresh loop) and explain safety/guardrail behavior.
• Add a practical "How to add a new collector" checklist plus repo-specific best practices and anti-patterns.

Validation

• go test ./... passed.
• go build ./... passed.