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/skill.go

@@ -0,0 +1,55 @@
+// internal/skills/debug/skill.go
+package debug
+
+import (
+	"context"
+	"encoding/json"
+
+	iexec "github.com/mathiasbq/supervisor/internal/exec"
+	"github.com/mathiasbq/supervisor/internal/registry"
+)
+
+// ExecutorFn is the function signature for running a worker subprocess.
+type ExecutorFn func(ctx context.Context, req iexec.Request) (iexec.Result, error)
+
+// Config holds dependencies for the debug skill.
+type Config struct {
+	SkillPrompt  string
+	DefaultModel string
+	ExecutorFn   ExecutorFn
+	SessionsDir  string
+}
+
+// Skill implements the debug MCP tool.
+type Skill struct{ cfg Config }
+
+// New creates a new debug Skill.
+func New(cfg Config) *Skill { return &Skill{cfg: cfg} }
+
+// Name returns the skill identifier.
+func (s *Skill) Name() string { return "debug" }
+
+// Tools returns the MCP tool definitions for this skill.
+func (s *Skill) Tools() []registry.ToolDef {
+	schema := func(required []string, props map[string]any) json.RawMessage {
+		b, _ := json.Marshal(map[string]any{"type": "object", "required": required, "properties": props})
+		return b
+	}
+	str := map[string]any{"type": "string"}
+	return []registry.ToolDef{
+		{
+			Name:        "debug",
+			Description: "Analyse an error and return 3-5 hypotheses ordered by likelihood, each with a concrete verification step.",
+			InputSchema: schema(
+				[]string{"project_root", "error"},
+				map[string]any{
+					"project_root": str,
+					"error":        str,
+					"context":      str,
+					"model":        str,
+					"session_id":   str,
+				},
+			),
+		},
+	}
+}