mara's ruling on this PR: the principle that landed for the role path is
general, so the no-role path moves the task out of the system prompt as
well. `system_prompt_and_trigger`'s no-role branch still passed
`prompt_file` straight through as `--append-system-prompt-file` —
byte-identical to main's pre-PR behaviour, and the exact bug this PR
exists to fix. Since no agent ships roles yet, every real dispatch takes
this path, so the standing-system-prompt bug (a task re-asserting itself
as an instruction on turn 2/3/N under continue/goal) was still live for
100% of usage; only the unused role path had actually been fixed.
`compose_prompt` now always folds the task into `trigger`, role or not.
Without a role there is no role text to hold a system-prompt file open
for, so none exists at all — `--append-system-prompt-file` is omitted
from the spawn entirely, not pointed at anything task-shaped.
Replaces `without_a_role_the_task_file_is_what_claude_is_pointed_at`
(which pinned the bug as "unchanged") with a test asserting the task is
absent from what reaches `--append-system-prompt-file` and present in
the trigger. Role-path tests are untouched in behavior; only mechanical
fallout from `compose_prompt`'s `Option<PathBuf>` return and `&str`
trigger params (clippy needless_pass_by_value once both branches only
borrowed it).
Blast radius: this changes behaviour for every existing dispatch, since
prompt_file's content has always gone into the system-prompt file before
this fix.
85ac62968e