feat(task): read descriptions from piped stdin

[birdmanmandbir]

Summary

• read piped stdin as the description for task add and bare task modify when no description text was supplied as CLI args
• trim trailing stdin whitespace like FlickNote so heredoc final newlines are not stored
• keep positional descriptions higher priority than piped stdin
• keep stdin available for modify confirmation prompts when modification args are present
• document the pipe forms in help and the man page

Verification

• make -C build-test task_executable -j4
• TASK_USE_PATH=1 PATH="$PWD/build-test/src:$PATH" python3 test/add.test.py
• TASK_USE_PATH=1 PATH="$PWD/build-test/src:$PATH" python3 test/modify.test.py -k TestModifyDescriptionInput
• manual bulk confirmation check: 1 2 modify priority:H with input="All\n" keeps descriptions unchanged and applies priority
• compared against FlickNote stdin behavior in flicknote-cli/src/commands/add.rs, modify.rs, replace.rs, and util.rs

Note: full test/modify.test.py still has an unrelated existing failure in TestBug1306: project:PROJ add foo exits with Project not found: PROJ in this fork.

[birdmanmandbir]

Triage Update

Fixed

• Bulk piped modify exhausted stdin before confirmation — addressed in commit 7cbb3a995; applyPipedDescriptionInput() now returns whether it consumed stdin, and CmdModify sets _permission_all for that piped-description path so bulk confirmation is not read from exhausted stdin.
• Missing bulk piped-description test — covered in test/modify.test.py by test_modify_bulk_description_from_pipe_auto_confirms, which runs 1 2 modify with rc.bulk=2 and piped description input.

False Positive

• None.

Deferred

• None.

Remaining

• None from this review.