fix: expose docs deep-dive routes in sidebar

[scaleborg]

Summary

• Add 6 deep-dive documentation pages as sub-items under Docs > How It Works
• Extend Docs header active state to highlight on deep-dive routes
• Add mcp_ecosystem ViewId and pathname detection for /how-mcp-works

Routes added to sidebar

• Ingestion Pipeline → /how-ingestion-works
• Chat Pipeline → /how-chat-works
• Career Intelligence → /how-career-works
• Observability → /how-monitor-works
• Architecture Reference → /how-architecture-works
• MCP Ecosystem → /how-mcp-works

Not changed

• /how-gpu-works intentionally left in Reference > GPUs (different domain)
• No router changes - all routes already existed
• No other sidebar sections modified

Test plan

• npx tsc --noEmit - 0 errors
• pnpm lint - 0 errors
• Playwright: all 6 routes navigate correctly from sidebar
• Playwright: SubNavItem shows [active] on each route
• Playwright: Docs header data-active="true" strictly verified on all 6

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