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/skill.go

@@ -0,0 +1,49 @@
+// internal/skills/sessionlog/skill.go
+package sessionlog
+
+import (
+	"encoding/json"
+
+	"github.com/mathiasbq/supervisor/internal/registry"
+)
+
+// Config holds sessionlog skill configuration.
+type Config struct {
+	SessionsDir string // path to brain/sessions/
+}
+
+// Skill implements registry.Skill for the session_log tool.
+type Skill struct {
+	cfg Config
+}
+
+// New constructs a sessionlog Skill.
+func New(cfg Config) *Skill { return &Skill{cfg: cfg} }
+
+// Name returns the skill name.
+func (s *Skill) Name() string { return "sessionlog" }
+
+// Tools returns the MCP tool definitions.
+func (s *Skill) Tools() []registry.ToolDef {
+	return []registry.ToolDef{
+		{
+			Name:        "session_log",
+			Description: "Append a structured entry to the current session log. Call after each skill invocation completes to record what happened for retrospective and training data extraction.",
+			InputSchema: json.RawMessage(`{
+				"type": "object",
+				"required": ["session_id"],
+				"properties": {
+					"session_id":   {"type": "string"},
+					"skill":        {"type": "string"},
+					"phase":        {"type": "string"},
+					"project_root": {"type": "string"},
+					"final_status": {"type": "string"},
+					"file_path":    {"type": "string"},
+					"model_used":   {"type": "string"},
+					"duration_ms":  {"type": "integer"},
+					"message":      {"type": "string"}
+				}
+			}`),
+		},
+	}
+}