Fix subagent context reporting

[nleononline]

Summary

• register SubagentStart and report the child agent's effective context-window limit
• rely exclusively on the hook-provided transcript_path and remove the unsafe session_id fallback
• update documentation and prepare patch release v0.2.1

Verification

• cargo fmt --check
• cargo test (16 passed)
• cargo clippy --all-targets --all-features -- -D warnings
• hook and plugin manifest JSON validation
• end-to-end SubagentStart probe with fork_turns="none"