v0.1.16

feat: count sub-agent tokens from subagent.completed events

Parse subagent.completed events in session-state JSONL to capture
sub-agent token usage for sessions without shutdown. This recovers
~13.3M previously uncounted tokens across 3 sessions on this machine.

Key changes:
- Add subagent_rows to CopilotFileDetails for cache persistence
- Parse subagent.completed events (model + totalTokens) in copilot.rs
- Prevent double-counting: clear seg_subagent on shutdown (modelMetrics
  already includes sub-agent usage), only trailing segment tokens added
- Include subagent_rows in OTel merge delta base to prevent double-count
  with OTel chat spans that also contain sub-agent calls
- Bump STATS_CACHE_VERSION 5→6 to invalidate old caches
- Add 4 unit tests: no-shutdown counting, shutdown no-double-count,
  null-field skip, OTel merge no-double-count

新增解析 subagent.completed 事件，捕获无 shutdown 会话的子 agent token。
关键改动：
- CopilotFileDetails 新增 subagent_rows 字段
- 解析 subagent.completed 的 model 和 totalTokens
- 防重复计数：shutdown 时清零 seg_subagent（modelMetrics 已含子 agent）
- OTel 合并时将 subagent_rows 纳入 delta 基准，防止与 chat span 重复
- 缓存版本 5→6
- 新增 4 个单元测试

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>