Skill: Add a new tasks-app command, end to end

A reusable playbook. Don't paste this whole file into a chat and hope. Point your agentic tool at it by name ("follow add-command.md to add a clear command"), or drop it wherever your tool auto-discovers procedures (a skills/commands folder). The steps are the same either way.

When to use this

Invoke this whenever the task is "add a new subcommand to the tasks-app CLI." It exists so a new command lands the same way every time: real code, a real test, a changelog line, and a clean commit; never just the code with the rest forgotten.

If the task is not "add a CLI command" (a bug fix, a refactor, a docs change), this skill does not apply. Don't force it.

Inputs you need before starting

Ask for these if they weren't given:

COMMAND_NAME: the subcommand word, e.g. clear.
WHAT_IT_DOES: one sentence of intended behavior, e.g. "remove all tasks."

Project facts (so you don't have to rediscover them)

Core logic lives in tasks.py (the TaskList class). The CLI front end is cli.py. State persists to tasks.json. Never edit tasks.json by hand; it's generated.
Tests live in test_tasks.py and run with python3 -m unittest. Standard library only; no third-party packages, no new dependencies.
The human-facing change log is CHANGELOG.md, newest entry on top.

Procedure: do these in order, do not skip

Core logic in tasks.py. If the command needs new behavior on the task list, add a small method to TaskList (e.g. clear()). Keep it minimal; match the existing style. If the command only reads existing state, skip to step 2.
Wire the CLI in cli.py. Add a branch to main() for COMMAND_NAME, call into tasks.py, save() if it mutated state, and print a short confirmation. Add the command to the usage: string so it's discoverable.
Add a real test in test_tasks.py. Test the behavior you intended, not just "it doesn't crash." Assert the end state (e.g. after clear(), len(tasks) == 0 and pending() is empty). A test that passes against a broken implementation is worse than no test.
Run the tests. python3 -m unittest from the project root. Do not claim success until it's green. If it fails, fix the code, not the test, and run again.
Smoke-test the CLI. Actually run it: python3 cli.py COMMAND_NAME, then python3 cli.py list to confirm the visible result. Paste what you ran and what it printed.
Add a CHANGELOG.md entry. One line under the top heading, present tense: - Add \COMMAND_NAME` command: WHAT_IT_DOES.` Newest on top.
Commit as one logical change. Stage code + test + changelog together and commit with a message that names the command: git add tasks.py cli.py test_tasks.py CHANGELOG.md && git commit -m "Add COMMAND_NAME command". Do not stage tasks.json.

Done when

python3 -m unittest is green and includes a new test that actually exercises COMMAND_NAME.
python3 cli.py COMMAND_NAME does WHAT_IT_DOES and you've shown the output.
CHANGELOG.md has a new top line for the command.
One commit contains the code, the test, and the changelog line, and nothing else (no tasks.json, no unrelated reformatting).

If any of those is missing, the skill isn't finished. Report which step failed and stop; don't paper over it.

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