Add service layer and MCP server with read tools

Slice 1: internal/service is the one capability layer both the HTTP API and the MCP server call (AGENT.md 1.7); /api/repos and /api/repo route through it. Slice 2: internal/mcp serves an MCP server over Streamable HTTP at /mcp (go-sdk v1.8.0) with read tools list_repos and get_repo as thin adapters over the service, plus a round-trip test. Enabled air polling so hot reload works across the Docker-on-Windows bind mount.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

go.mod

@@ -5,16 +5,23 @@ go 1.26
 require (
 	github.com/joho/godotenv v1.5.1
 	github.com/labstack/echo/v4 v4.15.4
+	github.com/modelcontextprotocol/go-sdk v1.8.0
 )
 
 require (
+	github.com/google/jsonschema-go v0.4.3 // indirect
 	github.com/labstack/gommon v0.5.0 // indirect
 	github.com/mattn/go-colorable v0.1.15 // indirect
 	github.com/mattn/go-isatty v0.0.22 // indirect
+	github.com/segmentio/asm v1.1.3 // indirect
+	github.com/segmentio/encoding v0.5.4 // indirect
 	github.com/valyala/bytebufferpool v1.0.0 // indirect
 	github.com/valyala/fasttemplate v1.2.2 // indirect
+	github.com/yosida95/uritemplate/v3 v3.0.2 // indirect
 	golang.org/x/crypto v0.53.0 // indirect
 	golang.org/x/net v0.56.0 // indirect
+	golang.org/x/oauth2 v0.35.0 // indirect
+	golang.org/x/sync v0.21.0 // indirect
 	golang.org/x/sys v0.46.0 // indirect
 	golang.org/x/text v0.38.0 // indirect
 	golang.org/x/time v0.15.0 // indirect