feat(cli): link a fujimoto new session to the session that started it

[jongracecox]

Summary

fujimoto new --parent NAME lets a Claude session that spins off work in a new worktree stay in touch with it, using Claude Code's built-in cross-session messaging (ListAgents / SendMessage).

• --parent NAME records the starting session's peer address as parent_session in the worktree's .fujimoto/meta.json. This happens on create and on connect, so the latest caller wins.
• The new session's system prompt names that address and tells it to use SendMessage to reach it.
• On resume, that one sentence is passed as the system prompt. An appended system prompt does not carry over into a new claude process, so without it a resumed child would lose its parent.
• --detach JSON gains parent_session.

The address is the peer name, not a Claude session id. I checked by experiment that SendMessage rejects a session id. A child's own address can be fixed up front with claude's --name after --. That is saved as a launch option, so it stays stable across relaunches. Claude Code's derived names (<dir>-<2 hex>) can't be predicted.

Deliberately only the mechanics: what a child should report, and when, is left to the prompt it is started with.

Test plan

• uv run pytest: 1145 passed. New tests cover meta round-trip and malformed values, --parent passed through, recorded on create and connect, worktree system prompt with and without a parent, resume restates the parent, detach JSON.
• End to end with this branch's build: a child launched with --parent messaged the parent using only its system prompt (its first message did not mention the address). The parent then messaged the child at its --name address and got a reply.

🤖 Generated with Claude Code