feat: add tier and session_log MCP tools

Adds two new MCP skill packages:
- internal/skills/org: exposes the tier tool, calling an injected TierFn
  for testability; returns current operating tier as structured JSON
- internal/skills/sessionlog: exposes the session_log tool, appending
  structured JSONL entries to brain/sessions/{session_id}.jsonl; requires
  session_id, wraps internal/session.Append

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

internal/skills/sessionlog/handlers.go

@@ -0,0 +1,54 @@
+// internal/skills/sessionlog/handlers.go
+package sessionlog
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"time"
+
+	"github.com/mathiasbq/supervisor/internal/session"
+)
+
+type logArgs struct {
+	SessionID   string `json:"session_id"`
+	Skill       string `json:"skill"`
+	Phase       string `json:"phase,omitempty"`
+	ProjectRoot string `json:"project_root,omitempty"`
+	FinalStatus string `json:"final_status,omitempty"`
+	FilePath    string `json:"file_path,omitempty"`
+	ModelUsed   string `json:"model_used,omitempty"`
+	DurationMs  int64  `json:"duration_ms,omitempty"`
+	Message     string `json:"message,omitempty"`
+}
+
+// Handle dispatches the session_log tool call.
+func (s *Skill) Handle(ctx context.Context, tool string, args json.RawMessage) (json.RawMessage, error) {
+	if tool != "session_log" {
+		return nil, fmt.Errorf("unknown sessionlog tool: %s", tool)
+	}
+	var a logArgs
+	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")
+	}
+
+	entry := session.Entry{
+		SessionID:   a.SessionID,
+		Timestamp:   time.Now().UTC(),
+		Skill:       a.Skill,
+		Phase:       a.Phase,
+		ProjectRoot: a.ProjectRoot,
+		FinalStatus: a.FinalStatus,
+		FilePath:    a.FilePath,
+		ModelUsed:   a.ModelUsed,
+		DurationMs:  a.DurationMs,
+	}
+	if err := session.Append(s.cfg.SessionsDir, a.SessionID, entry); err != nil {
+		return nil, fmt.Errorf("append session log: %w", err)
+	}
+	b, _ := json.Marshal(map[string]string{"status": "ok", "session_id": a.SessionID})
+	return b, nil
+}