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.go

@@ -0,0 +1,80 @@
+// internal/skills/trainer/handlers.go
+package trainer
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+
+	iexec "github.com/mathiasbq/supervisor/internal/exec"
+	"github.com/mathiasbq/supervisor/internal/session"
+)
+
+type trainArgs struct {
+	SessionID string `json:"session_id"`
+	Model     string `json:"model"`
+}
+
+// Handle dispatches the MCP tool call to the trainer handler.
+func (s *Skill) Handle(ctx context.Context, tool string, args json.RawMessage) (json.RawMessage, error) {
+	if tool != "trainer" {
+		return nil, fmt.Errorf("unknown tool: %s", tool)
+	}
+	var a trainArgs
+	if err := json.Unmarshal(args, &a); err != nil {
+		return nil, fmt.Errorf("parse args: %w", err)
+	}
+	if a.SessionID == "" {
+		return nil, fmt.Errorf("session_id is required")
+	}
+	if s.cfg.ExecutorFn == nil {
+		return nil, fmt.Errorf("no executor configured")
+	}
+
+	model := a.Model
+	if model == "" {
+		model = s.cfg.DefaultModel
+	}
+
+	entries, err := session.Read(s.cfg.SessionsDir, a.SessionID)
+	if err != nil {
+		return nil, fmt.Errorf("read session log: %w", err)
+	}
+
+	// ── Step 1: Reader agent ─────────────────────────────────────────────────
+	history := session.FormatHistory(entries, "")
+	readerTask := fmt.Sprintf(
+		"role: reader\nsession_id: %s\nbrain_dir: %s\n\n%s",
+		a.SessionID, s.cfg.BrainDir, history,
+	)
+	readerResult, err := s.cfg.ExecutorFn(ctx, iexec.Request{
+		SkillPrompt: s.cfg.ReaderPrompt,
+		TaskPrompt:  readerTask,
+		Model:       model,
+		Tools:       "Read",
+	})
+	if err != nil {
+		return nil, fmt.Errorf("reader agent: %w", err)
+	}
+
+	// ── Step 2: Writer agent (receives reader candidates) ────────────────────
+	writerTask := fmt.Sprintf(
+		"role: writer\nsession_id: %s\nbrain_dir: %s\n\nreader_summary: %s\nreader_candidates:\n%s",
+		a.SessionID, s.cfg.BrainDir, readerResult.Message, readerResult.RunnerOutput,
+	)
+	writerResult, err := s.cfg.ExecutorFn(ctx, iexec.Request{
+		SkillPrompt: s.cfg.WriterPrompt,
+		TaskPrompt:  writerTask,
+		Model:       model,
+		Tools:       "Read,Write",
+	})
+	if err != nil {
+		return nil, fmt.Errorf("writer agent: %w", err)
+	}
+
+	b, err := json.Marshal(writerResult)
+	if err != nil {
+		return nil, fmt.Errorf("marshal result: %w", err)
+	}
+	return b, nil
+}

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)
+}

internal/skills/trainer/skill.go

@@ -0,0 +1,53 @@
+// internal/skills/trainer/skill.go
+package trainer
+
+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 trainer skill.
+type Config struct {
+	ReaderPrompt string
+	WriterPrompt string
+	DefaultModel string
+	ExecutorFn   ExecutorFn
+	SessionsDir  string
+	BrainDir     string // root of brain/ directory; writer writes to BrainDir/training-data/
+}
+
+// Skill implements the trainer MCP tool.
+type Skill struct{ cfg Config }
+
+// New creates a new trainer Skill.
+func New(cfg Config) *Skill { return &Skill{cfg: cfg} }
+
+// Name returns the skill identifier.
+func (s *Skill) Name() string { return "trainer" }
+
+// 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
+	}
+	return []registry.ToolDef{
+		{
+			Name:        "trainer",
+			Description: "Extract SFT and DPO training pairs from a session log. Runs a reader→writer chain: reader identifies learning moments, writer formats and writes pairs to brain/training-data/.",
+			InputSchema: schema(
+				[]string{"session_id"},
+				map[string]any{
+					"session_id": map[string]any{"type": "string"},
+					"model":      map[string]any{"type": "string"},
+				},
+			),
+		},
+	}
+}