feat(memory): implement token usage tracking (DC-MEM-005)

[radoxtech]

Summary

• Add token_usage table (migration 008) with session_id FK to sessions, turn_id linkage, agent_id, model, provider, tokens_in/out, cost_usd, and timestamp
• Implement TokenUsageRepository with write path (record), per-session/agent/model aggregation queries, time-range filtering, and budget threshold check (checkBudget)
• Add Zod schemas for TokenUsage, RecordTokenUsageInput, SessionUsageSummary, ModelUsageBreakdown, and AgentUsageSummary types
• Export all new types and repository from @diricode/memory

Schema

CREATE TABLE token_usage (
  id TEXT PRIMARY KEY,
  session_id TEXT NOT NULL REFERENCES sessions(id) ON DELETE CASCADE,
  turn_id TEXT,
  agent_id TEXT,
  model TEXT NOT NULL,
  provider TEXT,
  tokens_in INTEGER NOT NULL DEFAULT 0,
  tokens_out INTEGER NOT NULL DEFAULT 0,
  cost_usd REAL NOT NULL DEFAULT 0.0,
  timestamp TEXT NOT NULL DEFAULT (datetime('now'))
);
-- Indexes: (session_id, timestamp), (turn_id), (model), (agent_id)

API Contracts

Provides stable interfaces for:

• DC-SAFE-004 (cost guardrails): checkBudget(sessionId, threshold) for live budget enforcement
• DC-OBS-* (observability): getSessionTotals, getAgentTotals, getModelBreakdown for metrics

Closes #94