docs: expand Architecture section with actors and data flow diagram

[paradoxbound]

Summary

• Adds an Actors table listing all system participants (MCP client, server, BookStack API client, BookStack instance, operator)
• Adds an ASCII data flow diagram showing how requests travel through the system end-to-end
• Documents key design decisions (native fetch, stdio transport, Zod validation, write-gate, monorepo split)

Satisfies OSPS-SA-01.01 — project documentation must include design documentation demonstrating all actions and actors within the system.

Test plan

• README renders correctly on GitHub
• ASCII diagram is readable in plain text

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