feat(trainer): add trainer MCP skill with reader→writer sub-agent chain

Reader agent scans session logs for SFT/DPO candidates; writer receives
reader output and formats+writes training pairs to brain/training-data/.
Adds trainer-reader.md and trainer-writer.md discipline prompts.

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

internal/skills/trainer/handlers_test.go

@@ -0,0 +1,82 @@
+// internal/skills/trainer/handlers_test.go
+package trainer_test
+
+import (
+	"context"
+	"encoding/json"
+	"testing"
+
+	iexec "github.com/mathiasbq/supervisor/internal/exec"
+	"github.com/mathiasbq/supervisor/internal/session"
+	"github.com/mathiasbq/supervisor/internal/skills/trainer"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestTrainerToolRegistered(t *testing.T) {
+	sk := trainer.New(trainer.Config{ReaderPrompt: "r", WriterPrompt: "w"})
+	names := make([]string, 0)
+	for _, tool := range sk.Tools() {
+		names = append(names, tool.Name)
+	}
+	assert.Contains(t, names, "trainer")
+}
+
+func TestTrainerRequiresSessionID(t *testing.T) {
+	sk := trainer.New(trainer.Config{ReaderPrompt: "r", WriterPrompt: "w"})
+	_, err := sk.Handle(context.Background(), "trainer", json.RawMessage(`{}`))
+	assert.ErrorContains(t, err, "session_id")
+}
+
+func TestTrainerCallsReaderThenWriter(t *testing.T) {
+	sessDir := t.TempDir()
+	require.NoError(t, session.Append(sessDir, "sess-1", session.Entry{
+		SessionID: "sess-1", Skill: "tdd", Phase: "red", FinalStatus: "pass",
+		Message: "wrote failing test", FilePath: "internal/foo/foo_test.go",
+	}))
+
+	callCount := 0
+	var readerTask, writerTask string
+
+	fakeFn := func(_ context.Context, req iexec.Request) (iexec.Result, error) {
+		callCount++
+		if callCount == 1 {
+			// reader call
+			readerTask = req.TaskPrompt
+			return iexec.Result{
+				Status: "pass", Phase: "trainer", Skill: "trainer",
+				RunnerOutput: `[{"type":"sft","moment":"first-pass clean TDD","score":4}]`,
+				Verified: true, ModelUsed: "self", Message: "1 sft candidate found",
+			}, nil
+		}
+		// writer call
+		writerTask = req.TaskPrompt
+		return iexec.Result{
+			Status: "pass", Phase: "trainer", Skill: "trainer",
+			FilePath: sessDir + "/training-data/sft/sess-1.jsonl",
+			Verified: true, ModelUsed: "self", Message: "1 sft pair written",
+		}, nil
+	}
+
+	sk := trainer.New(trainer.Config{
+		ReaderPrompt: "reader rules",
+		WriterPrompt: "writer rules",
+		ExecutorFn:   fakeFn,
+		SessionsDir:  sessDir,
+		BrainDir:     t.TempDir(),
+	})
+	out, err := sk.Handle(context.Background(), "trainer", json.RawMessage(`{"session_id":"sess-1"}`))
+	require.NoError(t, err)
+
+	assert.Equal(t, 2, callCount, "executor must be called exactly twice: reader then writer")
+	assert.Contains(t, readerTask, "role: reader")
+	assert.Contains(t, readerTask, "sess-1")
+	assert.Contains(t, readerTask, "wrote failing test") // session history in reader prompt
+	assert.Contains(t, writerTask, "role: writer")
+	assert.Contains(t, writerTask, "sft candidate") // reader output passed to writer
+
+	var result iexec.Result
+	require.NoError(t, json.Unmarshal(out, &result))
+	assert.Equal(t, "trainer", result.Phase)
+	assert.Equal(t, "pass", result.Status)
+}