refactor(mcp): migrate server to official go-sdk

[pocky]

Summary

• Replaces the custom pkg/mcpserver JSON-RPC 2.0 implementation (~1270 LOC) with the official github.com/modelcontextprotocol/go-sdk, reducing protocol maintenance burden and enabling future spec extensions (F108)
• Wraps the SDK in a new internal/infrastructure/mcp/ adapter that preserves hexagonal architecture — the CLI layer only depends on domain ports, not SDK types
• Hardens related infrastructure: mcp_proxy_purge now distinguishes timeout/not-installed/error failure modes with actionable log levels, and mcp_serve.go treats a failed os.Getwd() as a fatal error rather than silently disabling the sandbox
• Documents the migration decision in ADR 019 and adds SDK-level integration tests that exercise the real transport layer

Changes

Deleted: Custom MCP server package

• pkg/mcpserver/server.go: Deleted — custom JSON-RPC 2.0 server implementation
• pkg/mcpserver/protocol.go: Deleted — custom wire-protocol types and constants
• pkg/mcpserver/types.go: Deleted — custom type definitions
• pkg/mcpserver/doc.go: Deleted — package documentation
• pkg/mcpserver/architecture_test.go: Deleted — AST import constraint tests
• pkg/mcpserver/protocol_test.go: Deleted — protocol serialization tests
• pkg/mcpserver/server_test.go: Deleted — server behavior tests

New: Official SDK adapter

• internal/infrastructure/mcp/server.go: RegisterProvider now uses a two-pass (validate-then-commit) pattern for atomicity; extracted serve() helper shared by ServeStdio and new ServeIO
• internal/infrastructure/mcp/handler.go: Added nil-result guard and malformed-JSON fallback; removed redundant err = nil in panic recovery
• internal/infrastructure/mcp/mapping.go: Removed schemaFromMap and jsonschema-go dependency; toolToMCP passes InputSchema raw map directly to SDK
• internal/infrastructure/mcp/doc.go: Updated to reflect removal of schemaFromMap and jsonschema-go import
• internal/infrastructure/mcp/architecture_test.go: Removed github.com/google/jsonschema-go/ from allowed import list
• internal/infrastructure/mcp/testhelpers_test.go: Extracted shared fakeProvider and recordingProvider test doubles (previously duplicated across test files)
• internal/infrastructure/mcp/mcp_test.go: Added TestServeIO_* parallel-safe tests using in-memory pipes; added atomicity assertion for duplicate-tool registration; documented SDK non-deterministic error behavior on context cancellation
• internal/infrastructure/mcp/handler_test.go: Refactored to use shared test doubles; added TestHandlerFor_NilResult, TestHandlerFor_NilParams, TestHandlerFor_MalformedJSONArgs
• internal/infrastructure/mcp/mapping_test.go: Removed TestSchemaFromMap_* tests (function deleted); cleaned up wantErr fields

CLI interface

• internal/interfaces/cli/mcp_serve.go: os.Getwd() failure is now a fatal ExitSystem error instead of silently leaving rootDir empty (sandbox bypass)
• internal/interfaces/cli/mcp_serve_plugin_test.go: Extracted requestToolsList helper; added TestArchitecture_MCPServe_NewUsesVersion (AST enforcement that inframcp.New(Version) is called); strengthened TestResolveOperationProvider_EmptyDepsInitializes assertions
• internal/interfaces/cli/mcp_serve_helpers_test.go: Deleted (empty file)

Infrastructure: MCP proxy purge

• internal/infrastructure/agents/mcp_proxy_purge.go: Extracted named constants (mcpListDefaultTimeout, mcpRemoveTimeout, exitCodeCommandNotFound); added resolveListTimeout with env-var override; distinguished timeout (WARN), execution error (DEBUG), exit-127 (DEBUG), and non-zero exit (DEBUG) failure modes; added firstLine helper for single-line stderr in logs
• internal/infrastructure/agents/mcp_proxy_purge_test.go: Added warnCalls to log capture; added TestResolveListTimeout, TestPurgeOrphanMCPRegistrations_TimeoutLogsWarn, TestPurgeOrphanMCPRegistrations_NotInstalledIsQuiet

Domain ports

• internal/domain/ports/tool_provider.go: Added doc comment to ToolProvider interface specifying nil/empty-args equivalence, non-nil success result requirement, and dual error-reporting contract

Integration tests

• tests/integration/mcp/sdk_client_test.go: New — end-to-end tests exercising the SDK transport layer with a real MCP client
• tests/integration/mcp/plugin_bridge_test.go: Minor import/assertion updates for SDK adapter

Documentation & config

• docs/ADR/019-mcp-server-sdk-adapter.md: New ADR documenting the migration decision, rationale, alternatives, and trade-offs
• docs/ADR/017-mcp-proxy-stdio-subprocess-for-tool-interception.md: Updated implementation note to reference F104/ADR 019
• docs/ADR/README.md: Added ADR 019 entry
• docs/development/architecture.md: Added mcp/ entry to infrastructure layer
• docs/reference/package-documentation.md: Added internal/infrastructure/mcp to documented packages (28 total)
• .go-arch-lint.yml: Removed go-jsonschema vendor and its canUse entry; added explanatory comment
• go.mod: Marked github.com/google/jsonschema-go as // indirect

Test plan

• make build && make lint pass with zero violations
• make test and make test-race pass, including new sdk_client_test.go integration tests
• awf mcp-serve subprocess correctly lists and invokes builtin tools over stdio when exercised via mcp_proxy in a real workflow run
• pkg/mcpserver has zero remaining importers: grep -r "pkg/mcpserver" --include="*.go" . returns no results

Closes #365

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