core: reference the DONE_MARKER constant in user-facing copy

[Huoyanlifusu]

Problem

Exercise.DONE_MARKER is defined in pythonlings/core/exercise.py, but the marker string is still written out literally in a few places:

• pythonlings/core/exercise.py:20 — DONE_MARKER = "# I AM NOT DONE" (the constant)
• pythonlings/cli.py:256 — hardcoded in the "marker still present" error message
• pythonlings/widgets/output_panel.py:104 — hardcoded in "Remove the # I AM NOT DONE line to advance."
• pythonlings/screens/welcome.py:19 — hardcoded in the welcome instructions

So the marker exists in three copies that are only kept in sync by convention. If the marker ever changes (e.g. a (v2) suffix to migrate the curriculum), the runner will recognize the new marker while the error/welcome copy still tells learners to look for the old one. Nothing fails loudly — the tests would stay green while learners get misleading instructions.

Proposed Behavior

Reference the constant instead of the literal in those three spots, e.g.:

f"pythonlings: tests pass but the {Exercise.DONE_MARKER!r} marker is still in {name}.\n"

and the equivalent for the welcome and output-panel copy. Keeps the user-visible text in sync with what the runner actually checks, at no behavioral change.

Alternatives Considered

• Leave as-is: works today, but it's exactly the kind of string coupling that quietly breaks on the next marker change.
• Move the copy strings next to the constant as well: more thorough, but the three messages are in different screens anyway, so referencing the constant is enough.

[Huoyanlifusu]

For context: rustlings dropped the marker entirely (see their CHANGELOG: "The comment 'I AM NOT DONE!' doesn't exist anymore... you need to enter n in the terminal"). But pythonlings' AGENTS.md explicitly preserves the # I AM NOT DONE completion marker, so keeping the string consistent is on us.