Adds a child-exit-code guard to handleProvisional so a `ctask new`
workspace is reclaimed only when the agent was actually canceled
before real work (trust prompt rejected, Esc during startup, Ctrl+C
mid-launch — all confirmed empirically as exit code 1). A zero exit
means the user entered the agent and exited cleanly, which is a
legitimate workflow that must preserve the workspace even with an
empty manifest diff — for example when the user wants the workspace
directory established so they can populate context/ before resuming.
The exit code reaches handleProvisional via a new childExitCode
helper that unwraps *exec.ExitError from cmd.Run's return. Non-exit
errors (agent not found, OS-level failure) map to -1 so the
workspace is still cleaned up — the child never actually ran.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>