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
+}