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/org/handlers.go

@@ -0,0 +1,21 @@
+// internal/skills/org/handlers.go
+package org
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+)
+
+// Handle dispatches the tier tool call.
+func (s *Skill) Handle(ctx context.Context, tool string, args json.RawMessage) (json.RawMessage, error) {
+	if tool != "tier" {
+		return nil, fmt.Errorf("unknown org tool: %s", tool)
+	}
+	info := s.cfg.TierFn(ctx)
+	b, err := json.Marshal(info)
+	if err != nil {
+		return nil, fmt.Errorf("marshal tier info: %w", err)
+	}
+	return b, nil
+}