Expose subagent tasks and token detail in checkpoint explain

checkpoint explain --json copied four token fields into its own struct, dropping subagent_tokens, subagent_tokens_complete and api_call_count, and nothing read back the tasks/<tool_use_id>/ records the writer has materialized since #2058.

Add checkpoint.TaskReader (ListTasks, ReadTaskTranscript) to the persistent store contract, implemented for both git backends by one shared tree reader that re-validates the pushed directory name and agent_id, forwarded by the routing and fan-out wrappers, stored by fsstore, and reported unsupported by the --repo API reader. --json now carries the full per-session TokenUsage and a top-level tasks array; unreadable records set partial. --transcript --task <tool_use_id|agent_id> streams one subagent's stored transcript.