Align API key configuration: OpenRouter:ApiKey with OPEN_ROUTER_API_KEY fallback

[Copilot]

The README documented OpenRouter:ApiKey (nested) for appsettings.json, but appsettings.json used a flat OPEN_ROUTER_API_KEY key and all 23 agent components read Configuration["OPEN_ROUTER_API_KEY"] — three sources out of sync.

Changes

• appsettings.json — switched to nested OpenRouter:ApiKey format to match README
• All 23 Agent razor files — updated API key lookup to prefer OpenRouter:ApiKey with fallback to OPEN_ROUTER_API_KEY for backwards compatibility; error message updated to reflect both

// Before
var apiKey = Configuration["OPEN_ROUTER_API_KEY"] ??
    throw new InvalidOperationException("OPEN_ROUTER_API_KEY is not set in.");

// After
var apiKey = Configuration["OpenRouter:ApiKey"] ?? Configuration["OPEN_ROUTER_API_KEY"] ??
    throw new InvalidOperationException("OpenRouter:ApiKey (or OPEN_ROUTER_API_KEY) is not configured.");

The README required no changes — it already documented both options correctly.

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