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_test.go

@@ -0,0 +1,61 @@
+// internal/skills/debug/handlers_test.go
+package debug_test
+
+import (
+	"context"
+	"encoding/json"
+	"testing"
+
+	iexec "github.com/mathiasbq/supervisor/internal/exec"
+	"github.com/mathiasbq/supervisor/internal/skills/debug"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestDebugToolRegistered(t *testing.T) {
+	sk := debug.New(debug.Config{SkillPrompt: "debug rules"})
+	names := make([]string, 0)
+	for _, tool := range sk.Tools() {
+		names = append(names, tool.Name)
+	}
+	assert.Contains(t, names, "debug")
+}
+
+func TestDebugRequiresProjectRoot(t *testing.T) {
+	sk := debug.New(debug.Config{SkillPrompt: "d"})
+	_, err := sk.Handle(context.Background(), "debug", json.RawMessage(`{"error":"panic: nil pointer"}`))
+	assert.ErrorContains(t, err, "project_root")
+}
+
+func TestDebugRequiresError(t *testing.T) {
+	sk := debug.New(debug.Config{SkillPrompt: "d"})
+	_, err := sk.Handle(context.Background(), "debug", json.RawMessage(`{"project_root":"/tmp"}`))
+	assert.ErrorContains(t, err, "error")
+}
+
+func TestDebugCallsExecutor(t *testing.T) {
+	called := false
+	var capturedTask string
+	fakeFn := func(_ context.Context, req iexec.Request) (iexec.Result, error) {
+		called = true
+		capturedTask = req.TaskPrompt
+		return iexec.Result{
+			Status: "pass", Phase: "debug", Skill: "debug",
+			RunnerOutput: "HYPOTHESIS 1 (likelihood: high): nil map access\nVERIFY: go test ./... → expected: panic line reference",
+			Verified:     false, ModelUsed: "self", Message: "3 hypotheses for: panic nil pointer at foo.go:42",
+		}, nil
+	}
+
+	sk := debug.New(debug.Config{SkillPrompt: "debug rules", ExecutorFn: fakeFn, SessionsDir: t.TempDir()})
+	out, err := sk.Handle(context.Background(), "debug", json.RawMessage(
+		`{"project_root":"/tmp/proj","error":"panic: nil pointer dereference at foo.go:42","context":"occurs on startup"}`,
+	))
+	require.NoError(t, err)
+	assert.True(t, called)
+	assert.Contains(t, capturedTask, "panic: nil pointer dereference")
+	assert.Contains(t, capturedTask, "occurs on startup")
+
+	var result iexec.Result
+	require.NoError(t, json.Unmarshal(out, &result))
+	assert.Equal(t, "debug", result.Phase)
+}