Add a `count` command that prints the number of pending tasks

[recklessop]

Context

list shows all tasks, but there's no quick way to answer "how much do I still have to do?" without eyeballing the checkbox characters. count gives you the answer in one number, which is useful for scripting and for a fast sanity check before starting a work session.

Behavior

$ python3 cli.py count
3 pending

Reads from tasks.json. Doesn't write. Exits 0 on success.

Acceptance criteria

• python3 cli.py count prints <N> pending where N excludes tasks marked done.
• Empty list prints 0 pending, exits 0.
• Usage line includes count alongside add | list | done.
• New tests in tests/test_tasks.py cover: none pending, some pending, all done.
• No dependency additions (stdlib only).

Not in scope

• Category filters, tags, or due-date filtering. Just the count.
• Coloring the output. Plain text only.

Files that may change

• tasks.py — add count_pending(self) -> int, reusing the existing pending() helper.
• cli.py — add elif command == "count": branch and update the usage string.
• tests/test_tasks.py — new test cases for the three scenarios above.