* fix(tui): report headless --print/--json failures instead of leaking tracebacks
The headless --print and --json runners dispatched straight into the
embedded client with no error boundary, so session-creation, thread-
resolution, chat, and stream failures escaped main() and surfaced as
unhandled Python tracebacks from the console script - for example the
documented CheckpointModeMismatchError case, which the checkpoint
operations/troubleshooting docs recorded as a raw traceback.
Both runners now catch Exception (KeyboardInterrupt/SystemExit still
propagate), mirroring the TUI's stream_actions() boundary:
- --print writes one concise 'Error: ...' line to stderr and returns 1
- --json appends a terminal {"type": "error"} record to the NDJSON
stream (diagnostics stay on stderr) and returns 1; every stdout line
remains independently valid JSON, including failures mid-stream
The checkpoint operations/troubleshooting docs (en + zh) no longer
describe the headless runners as printing an uncaught traceback.
Fixes#5962
* fix(tui): close the broken-pipe edge and document the --json error contract
Review follow-ups on #5963 (both non-blocking):
- print(answer) sat outside the headless boundary, so a consumer closing
the pipe early (deerflow --print ... | head) still got a
BrokenPipeError traceback plus the interpreter's shutdown-flush noise.
The final --print write and the --json terminal error-record write now
catch BrokenPipeError, point the stdout descriptor at devnull (the
documented SIGPIPE-adjacent recipe) and exit 1 without a traceback.
- The terminal {"type": "error"} record is a real contract for --json
scripters but was only documented in the checkpoint pages; the
backend/docs/TUI.md command table and the README cheat line now state
the error record and the exit-1 semantics where CLI consumers look.
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