Let the atomic Planner hand an implement request to another session over Intercom

The Planner does not implement, so a member who asked it to implement
the plan now got nothing useful back. Under HARNESS=atomic the Planner
has an `intercom` tool, which reaches other sessions on the same
machine, including one working in a checkout of the document's
repository, such as the session that handed it the document. Its
instructions now say it can pass the request to such a session and tell
the member where the work continues, or that no session could take it.
Isolated harnesses are unchanged.

Assistant-model: Claude Opus 5.5
Assistant-workflow: inline
Assistant-verification: bun test passed: session and agent tests (142 pass), including the new instruction text for checkout and scratch workspaces and its absence for isolated harnesses
Assistant-verification: bun run types passed: all workspaces
Assistant-verification: bun run ci passed: dprint, oxlint, tokens, design contract, Impeccable (no new findings)
User-preference: When a member asks to implement the plan, the atomic Planner should hand off to another session through its intercom tool; keep the guidance general and not overly prescriptive
User-preference: Talk generally about the `intercom` tool and other sessions rather than "Atomic sessions"
Co-authored-by: Alex Lavaee <lavaman131@github.com>