fix(opencode): use file times for truncation cleanup

[opencode-agent]

Summary

• use truncation file modification time for retention cleanup
• stop deriving file age from wrapping tool IDs
• cover cleanup across the identifier wrap boundary

Testing

• bun test test/tool/truncation.test.ts --timeout 30000 --only-failures (19 passed in implementation worktree)
• Prettier check
• git diff --check

Requested by: @thdxr (Dax Raad via Slack)