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/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"},
+				},
+			),
+		},
+	}
+}