docs: seed _context/wiki with full architecture book coverage

[Lang-Akshay]

What

Expands _context/wiki/ from 3 stub pages to 8 targeted reference pages drawn from the full docs/book/src/ architecture book.

Pages added

File	Covers
architecture.md	Middleware stack order, pipeline shape, module boundaries, state lifetime, executor shapes
routing.md	Backend prefix contract, list/routed ops, federated pagination, session state, capability merge, MCP method quick reference
failure-modes.md	Complete HTTP/MCP/routing/backend/plugin/Redis failure table
config.md	CLI flags, JWT claims, UserConfig shape, protected headers, plugin config, telemetry debugging, startup validation
deployment.md	Deployment checklist, health endpoint caveat, nginx routing, session affinity, Redis availability, image pinning, security posture

Pages updated

• preferences.md — full validation gate (cargo deny, cargo build, CI cargo shear); by-change-type table
• index.md — updated page table and quick-orientation validation gate note

What was kept out

Section-index pages (operations.md, architecture.md, usage.md, mcp-behavior.md) and one-time setup content (publishing-this-book.md, resource budgets, mcp-inspector) — low reuse value for agents.

[lucarlig]

We need an unique source of truth. We already have the book and the Summary.md please optimize that for AI rather than writing something new from scratch