Launch incubator sessions through the per-connection monitor, passing
stdio and the optional environment pipe via SCM_RIGHTS. tailscaled
retains the parent I/O descriptors. Keep direct launch when no
tailscaled binary is available, and on Plan 9.
Wait for dispatched exec replies even after session cancellation so
the session retains ownership of any child that starts. Publish exit
status through persistent completion state. Identify monitor children
by PID and exec request ID, and validate both when signaling. Remove
raw-PID signaling after monitor teardown.
Normalize incubator identity flags and reject alternate spellings
that could override them. The monitor RPC caller remains trusted to
supply the executable and other arguments; this is not a privilege
boundary against tailscaled.
Pass default TERM and locale values through the environment pipe,
preserving last-value-wins defaults and policy-forwarded precedence.
Use a separately owned close-on-exec PTY descriptor for resizing.
Add regression tests for launch cancellation, exit delivery, signal
identity, descriptor ownership, and environment handling.
Updates tailscale/corp#50380
Signed-off-by: James Tucker <james@tailscale.com>
