feat(debug): add debug MCP skill with hypothesis generation

Implements the debug skill following the same pattern as review. The skill
accepts project_root + error (+ optional context/model/session_id), prepends
session history, and calls the executor to produce 3-5 ordered hypotheses —
diagnosis only, no fixes.

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

internal/skills/debug/handlers.go

@@ -0,0 +1,80 @@
+// internal/skills/debug/handlers.go
+package debug
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+
+	iexec "github.com/mathiasbq/supervisor/internal/exec"
+	"github.com/mathiasbq/supervisor/internal/session"
+)
+
+type debugArgs struct {
+	ProjectRoot string `json:"project_root"`
+	Error       string `json:"error"`
+	Context     string `json:"context"`
+	Model       string `json:"model"`
+	SessionID   string `json:"session_id"`
+}
+
+// Handle dispatches the MCP tool call to the appropriate handler.
+func (s *Skill) Handle(ctx context.Context, tool string, args json.RawMessage) (json.RawMessage, error) {
+	if tool != "debug" {
+		return nil, fmt.Errorf("unknown tool: %s", tool)
+	}
+	var a debugArgs
+	if err := json.Unmarshal(args, &a); err != nil {
+		return nil, fmt.Errorf("parse args: %w", err)
+	}
+	if a.ProjectRoot == "" {
+		return nil, fmt.Errorf("project_root is required")
+	}
+	if a.Error == "" {
+		return nil, fmt.Errorf("error is required")
+	}
+
+	model := a.Model
+	if model == "" {
+		model = s.cfg.DefaultModel
+	}
+
+	task := fmt.Sprintf(
+		"phase: debug\nproject_root: %s\nerror: %s\ncontext: %s\nmodel: %s",
+		a.ProjectRoot, a.Error, a.Context, model,
+	)
+	task = s.prependHistory(a.SessionID, "debug", task)
+
+	if s.cfg.ExecutorFn == nil {
+		return nil, fmt.Errorf("no executor configured")
+	}
+	result, err := s.cfg.ExecutorFn(ctx, iexec.Request{
+		SkillPrompt: s.cfg.SkillPrompt,
+		TaskPrompt:  task,
+		Model:       model,
+		Tools:       "Read,Bash",
+	})
+	if err != nil {
+		return nil, err
+	}
+	b, err := json.Marshal(result)
+	if err != nil {
+		return nil, fmt.Errorf("marshal result: %w", err)
+	}
+	return b, nil
+}
+
+func (s *Skill) prependHistory(sessionID, currentPhase, task string) string {
+	if sessionID == "" || s.cfg.SessionsDir == "" {
+		return task
+	}
+	entries, err := session.Read(s.cfg.SessionsDir, sessionID)
+	if err != nil || len(entries) == 0 {
+		return task
+	}
+	history := session.FormatHistory(entries, currentPhase)
+	if history == "" {
+		return task
+	}
+	return history + "\n---\n\n" + task
+}