feat(spec): add spec writing MCP skill

Adds the spec skill that generates structured implementation specs from
requirements and writes them to a configurable output path in the project.
Follows the same pattern as review/debug skills with session history injection.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

cmd/supervisor/main.go

@@ -15,6 +15,7 @@ import (
 	"github.com/mathiasbq/supervisor/internal/skills/retrospective"
 	skilldebug "github.com/mathiasbq/supervisor/internal/skills/debug"
 	"github.com/mathiasbq/supervisor/internal/skills/review"
+	"github.com/mathiasbq/supervisor/internal/skills/spec"
 	"github.com/mathiasbq/supervisor/internal/skills/sessionlog"
 	"github.com/mathiasbq/supervisor/internal/skills/tdd"
 	"github.com/mathiasbq/supervisor/internal/tier"
@@ -65,6 +66,12 @@ func main() {
 		os.Exit(1)
 	}
 
+	specPrompt, err := os.ReadFile(cfg.ConfigDir + "/spec.md")
+	if err != nil {
+		logger.Error("read spec.md", "path", cfg.ConfigDir+"/spec.md", "err", err)
+		os.Exit(1)
+	}
+
 	executor := iexec.New(iexec.Config{
 		SystemPrompt:   string(systemPrompt),
 		LiteLLMBaseURL: cfg.LiteLLMBaseURL,
@@ -110,6 +117,12 @@ func main() {
 		ExecutorFn:   executor.Run,
 		SessionsDir:  cfg.SessionsDir,
 	}))
+	reg.Register(spec.New(spec.Config{
+		SkillPrompt:  string(specPrompt),
+		DefaultModel: models.Resolve("spec", ""),
+		ExecutorFn:   executor.Run,
+		SessionsDir:  cfg.SessionsDir,
+	}))
 
 	srv := mcp.NewServer(reg)
 	mux := http.NewServeMux()