Function print_dev_instance_collision 

fn print_dev_instance_collision(
    channel: &str,
    data_dir: &Path,
    socket_path: &str,
)

User-facing message for an actual second-instance collision (not a bind-failure, not a stale socket — a real running launcher answered our connect probe). Shared by both call sites in bind_socket_with_recovery (fast-path and post-recovery-lock retry), which previously duplicated this message and both got it wrong the same way — see docs/specs/SPEC_DEV_INSTANCE_ISOLATION_DIAGNOSTICS_2026_09_19.md.

For a dev- channel, this used to say “Use task dev:local to launch a second isolated session” — plausible-sounding, and wrong for the common case. task dev:local’s version bump only changes the launcher’s own IPC socket key (hash.rs’s data_dir_hash16, keyed on CARGO_PKG_VERSION/AGENTMUX_BUILD_LABEL at compile time) — it does NOT change the CEF profile/data directory, which for dev mode is keyed on (branch, clone_id) — NOT version — via crates/common/src/data_paths.rs’s resolve_channel_and_dir (honor_env_channel=false for dev launches). clone_id (runtime_mode.rs’s derive_clone_id) is a hash of the CLONE’S OWN CANONICAL WORKSPACE-ROOT PATH — git-worktree-aware by design, so a second worktree of the SAME branch already gets a distinct clone_id and therefore a distinct data dir, with no branch switch needed; resolve_channel_and_dir folds both into the one channel string this function receives (format!("dev-{branch}-{clone_id}") — confirmed by reading that call site directly). Two dev instances collide on CEF’s own SingletonLock only when BOTH branch and clone/workspace path are identical — reproduced live: a real rebuild under a bumped version, run from the unchanged clone path on the unchanged branch, got a new launcher socket and STILL hit CEF early exit (process singleton or similar) on the unchanged data dir. The fix is a different branch OR a different clone/worktree path (either one changes channel) — named directly here instead of pointing at a command that cannot solve this specific problem. (ReAgent P1 on this PR: an earlier revision of this message claimed branch was the ONLY lever, which is what this revision corrects — verified against resolve_channel_and_dir’s actual source, not just re-asserted.)