fix(acp): show ACP goodbye card session stats

[CSRessel]

Summary

• Track ACP goodbye-card stats from normalized ClientEvent data: assistant replies, completed/failed tool snapshots, skill paths, and generic Agent tool snapshots.
• Add narrow transcript fallback for subagent names when a discovered JSONL transcript contains delegated-agent fields that are not visible as ACP tool events.
• Document the goodbye-card stats contract and update snapshots for the corrected assistant count.

Test Plan

• cargo test -p nori-tui
• cargo build -p mock-acp-agent
• cargo test -p nori-acp
• cargo build --bin nori
• cargo test -p tui-pty-e2e
• RUSTC_WRAPPER= just fix -p nori-tui
• RUSTC_WRAPPER= just fix -p nori-acp
• tmux close-loop: launched nori --agent elizacp --skip-trust-directory inside a persistent bash tmux session, submitted hello, exited with /exit, and verified Goodbye! plus Assistant: 1 on the card.

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