feat(framework): add first-class mcpServers field to AgentInvocation

[jafreck]

Summary

Replace the untyped extensions bag with a typed mcpServers field on AgentInvocation. Both CopilotBackend and ClaudeBackend now iterate mcpServers and emit the appropriate CLI flags (--additional-mcp-config / --mcp-config).

Changes

packages/framework/src/runtime/context/types.ts

• AgentInvocation: replaced extensions?: Record<string, unknown> with mcpServers?: Record<string, { url: string }> — typed, explicit, self-documenting
• AgentResult: removed unused extensions field

packages/framework/src/runtime/launch-with-events.ts

• InvocationMetric: removed unused extensions field

packages/framework/src/runtime/backend/backend.ts

• CopilotBackend.invoke(): iterates invocation.mcpServers and pushes --additional-mcp-config per server
• ClaudeBackend.invoke(): iterates invocation.mcpServers and pushes --mcp-config per server (with type: 'url' wrapper)

Motivation

The extensions field was a speculative untyped escape hatch (Record<string, unknown>) that was never set or read by any code — not by cadre itself, not by any consumer. Making MCP server forwarding a first-class typed field:

• Provides autocomplete and compile-time checking for consumers
• Eliminates blind as casts in the backends
• Keeps the framework free of consumer-specific concepts (no AAMF references)

[codecov]

Codecov Report

❌ Patch coverage is 0% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.32%. Comparing base (49b752b) to head (6d4bbc8).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
packages/framework/src/runtime/backend/backend.ts	0.00%	10 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #409      +/-   ##
==========================================
- Coverage   91.36%   91.32%   -0.04%     
==========================================
  Files         213      213              
  Lines       22862    22872      +10     
  Branches     3583     3585       +2     
==========================================
  Hits        20887    20887              
- Misses       1927     1937      +10     
  Partials       48       48              

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