Add top-level mcps section for reusable MCP server definitions

[dgageot]

Summary

Adds a top-level mcps section to the agent config for defining MCP server configurations once and referencing them by name from agent toolsets. This eliminates duplication when multiple agents use the same MCP server.

Before

agents:
  root:
    toolsets:
      - type: mcp
        ref: docker:context7
  frontend:
    toolsets:
      - type: mcp
        ref: docker:context7  # duplicated
  backend:
    toolsets:
      - type: mcp
        ref: docker:context7  # duplicated

After

mcps:
  context7:
    ref: docker:context7

agents:
  root:
    toolsets:
      - type: mcp
        ref: context7
  frontend:
    toolsets:
      - type: mcp
        ref: context7
  backend:
    toolsets:
      - type: mcp
        ref: context7

Override semantics

• Toolset fields take precedence over definition fields
• env maps are merged (toolset values win on key conflicts)
• Definitions support all MCP sources: command, remote, and ref (docker refs)

Changes

• pkg/config/latest/types.go — MCPToolset type (wraps Toolset, validates at parse time), MCPs field on Config
• pkg/config/latest/validate.go — Removed docker: prefix restriction from Toolset.validate() (now checked during MCP definition validation)
• pkg/config/mcps.go — Resolution logic: resolveMCPDefinitions, validateMCPDefinition, applyMCPDefaults
• pkg/config/config.go — Call resolveMCPDefinitions in validateConfig
• agent-schema.json — mcps property and MCPToolset definition
• examples/mcp-definitions.yaml — Example config
• pkg/config/mcps_test.go — 9 test cases

Closes #1667

[dgageot]

/review

[docker-agent]

Review Summary

Assessment: 🔴 CRITICAL

Found 2 confirmed bugs in the MCP definition resolution logic:

1. HIGH: The applyMCPDefaults function incorrectly clears the ref field for command-based definitions, losing the definition name
2. MEDIUM: The Remote struct merge logic only checks the URL field, which can cause Header values to be lost during override operations

Both issues are in newly added code and affect the core functionality of MCP definition resolution.