refactor: introduce IAegisBackend service layer for MCP (#1697)

[OneStepAt4time]

Summary

Introduces a shared service interface layer (IAegisBackend) between the MCP tools and the Aegis backend, enabling both remote (HTTP) and in-process (embedded) operation modes.

Changes

• src/services/interfaces.ts — New file defining 5 domain interfaces (ISessionService, IServerService, IPipelineService, IMemoryService, IAuthService) and their composite IAegisBackend, plus shared response types
• src/mcp/client.ts — AegisClient now implements IAegisBackend; response types moved to shared interfaces; fix \createPipeline\ body to send \stages\ (matching API schema)
• src/mcp/embedded.ts — New \EmbeddedBackend\ implementing \IAegisBackend\ via direct class calls (no HTTP round-trip)
• src/mcp/server.ts — New \createMcpServerFromBackend(backend)\ factory accepting any \IAegisBackend; existing \createMcpServer(port)\ delegates to it
• src/mcp/auth.ts, resources.ts, tools/*.ts — All accept \IAegisBackend\ instead of concrete \AegisClient\
• src/mcp-server.ts — Re-exports \EmbeddedBackend, \createMcpServerFromBackend, and \IAegisBackend\

Acceptance Criteria

• Service interfaces shared between HTTP routes and MCP
• MCP can operate without HTTP round-trip (in-process mode via EmbeddedBackend)
• AegisClient retained as remote-mode adapter
• All 24 MCP tools functional in both modes (interface-compatible)
• Quality gate: tsc, build, 159 test files / 2814 tests pass

Aegis version

Developed with: v0.3.2-alpha

Closes #1697