chore: drop the mcp-go dependency and remove mcpspike throwaway spike

Nothing in the backend speaks MCP anymore -- internal/garmin talks to its
embedded Python wrapper over plain JSON-lines instead. The cmd/mcpspike
directory was a temporary spike for validating the mcp-go client, which is
no longer needed.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/go.mod

@@ -6,7 +6,6 @@ require (
 	github.com/coreos/go-oidc/v3 v3.20.0
 	github.com/go-chi/chi/v5 v5.3.1
 	github.com/golang-jwt/jwt/v5 v5.3.1
-	github.com/mark3labs/mcp-go v0.56.0
 	golang.org/x/oauth2 v0.36.0
 	modernc.org/sqlite v1.53.0
 )
@@ -14,16 +13,11 @@ require (
 require (
 	github.com/dustin/go-humanize v1.0.1 // indirect
 	github.com/go-jose/go-jose/v4 v4.1.4 // indirect
-	github.com/google/jsonschema-go v0.4.2 // indirect
 	github.com/google/uuid v1.6.0 // indirect
 	github.com/mattn/go-isatty v0.0.20 // indirect
 	github.com/ncruces/go-strftime v1.0.0 // indirect
 	github.com/remyoudompheng/bigfft v0.0.0-20230129092748-24d4a6f8daec // indirect
-	github.com/santhosh-tekuri/jsonschema/v6 v6.0.2 // indirect
-	github.com/spf13/cast v1.7.1 // indirect
-	github.com/yosida95/uritemplate/v3 v3.0.2 // indirect
 	golang.org/x/sys v0.44.0 // indirect
-	golang.org/x/text v0.14.0 // indirect
 	modernc.org/libc v1.73.4 // indirect
 	modernc.org/mathutil v1.7.1 // indirect
 	modernc.org/memory v1.11.0 // indirect