docs(mcp): architecture overview and audit logging guide

[joshrotenberg]

Summary

• Add Architecture page covering server structure, feature flags, toolset/sub-module system, tool macros, transport modes, credential resolution, policy engine, audit layer, and request flow
• Add Audit Logging page covering configuration, audit levels, log format with JSON examples, argument redaction, RUST_LOG routing, log aggregation, and practical examples
• Update mkdocs nav, MCP index, and getting-started Next Steps with links to new pages

Closes #775 (remaining deliverables)

Test plan

• mkdocs serve renders both new pages correctly
• All internal links resolve (architecture.md, audit-logging.md cross-references)
• Nav entries appear in correct position under MCP section