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>

cmd/supervisor/main.go

@@ -16,6 +16,7 @@ import (
 	skilldebug "github.com/mathiasbq/supervisor/internal/skills/debug"
 	"github.com/mathiasbq/supervisor/internal/skills/review"
 	"github.com/mathiasbq/supervisor/internal/skills/spec"
+	"github.com/mathiasbq/supervisor/internal/skills/trainer"
 	"github.com/mathiasbq/supervisor/internal/skills/sessionlog"
 	"github.com/mathiasbq/supervisor/internal/skills/tdd"
 	"github.com/mathiasbq/supervisor/internal/tier"
@@ -72,6 +73,17 @@ func main() {
 		os.Exit(1)
 	}
 
+	trainerReaderPrompt, err := os.ReadFile(cfg.ConfigDir + "/trainer-reader.md")
+	if err != nil {
+		logger.Error("read trainer-reader.md", "path", cfg.ConfigDir+"/trainer-reader.md", "err", err)
+		os.Exit(1)
+	}
+	trainerWriterPrompt, err := os.ReadFile(cfg.ConfigDir + "/trainer-writer.md")
+	if err != nil {
+		logger.Error("read trainer-writer.md", "path", cfg.ConfigDir+"/trainer-writer.md", "err", err)
+		os.Exit(1)
+	}
+
 	executor := iexec.New(iexec.Config{
 		SystemPrompt:   string(systemPrompt),
 		LiteLLMBaseURL: cfg.LiteLLMBaseURL,
@@ -123,6 +135,14 @@ func main() {
 		ExecutorFn:   executor.Run,
 		SessionsDir:  cfg.SessionsDir,
 	}))
+	reg.Register(trainer.New(trainer.Config{
+		ReaderPrompt: string(trainerReaderPrompt),
+		WriterPrompt: string(trainerWriterPrompt),
+		DefaultModel: models.Resolve("trainer", ""),
+		ExecutorFn:   executor.Run,
+		SessionsDir:  cfg.SessionsDir,
+		BrainDir:     cfg.BrainDir,
+	}))
 
 	srv := mcp.NewServer(reg)
 	mux := http.NewServeMux()