Add API request reference docs: request/response architecture, clientName, x-github-request-id

[Copilot]

Documents how the SDK communicates with the Copilot API via the CLI, with deep dives on clientName and x-github-request-id.

New: docs/api-request-reference.md

• Architecture: SDK → JSON-RPC → CLI → HTTP → api.githubcopilot.com flow with diagrams
• Authentication: Token priority order (6 levels), supported token formats
• clientName: Data flow diagram, examples in all 4 languages for create/resume, JSON-RPC payload format, best practices
• x-github-request-id: Response header → SDK event field mapping, distinguishes providerCallId (from API header, for GitHub support debugging) vs requestId (CLI-internal, for tool/permission round-trips)
• Request/response format: OpenAI-compatible /chat/completions body, streaming SSE, BYOK provider differences
• Troubleshooting: Debug logging, token verification, providerCallId for support correlation

Key disambiguation — requestId has two meanings:

Context	Source	Purpose
session.error, session.usage_info → providerCallId	x-github-request-id HTTP header	Server-side log correlation
session.compaction_complete → requestId	x-github-request-id HTTP header	Same (historical naming)
permission.requested, external_tool.requested → requestId	CLI-generated UUID	Match request↔response callbacks

Updated: docs/index.md

Added API Request Reference to nav table and documentation map.

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