[Code scan] Resolve example input files relative to each script directory

This issue is a result of a Codex global code scan of `deepmodeling/fpop` at commit `b05b337590c31a5237b2dcbd9c0833b841c08cd4`.

Relevant code:
https://github.com/deepmodeling/fpop/blob/b05b337590c31a5237b2dcbd9c0833b841c08cd4/examples/vasp/preprunvasp.py#L67-L83
https://github.com/deepmodeling/fpop/blob/b05b337590c31a5237b2dcbd9c0833b841c08cd4/examples/cp2k/prepruncp2k.py#L67-L85
https://github.com/deepmodeling/fpop/blob/b05b337590c31a5237b2dcbd9c0833b841c08cd4/examples/abacus/preprunvasp.py#L67-L83

Problem:
The example scripts refer to bundled input files with current-working-directory-relative paths such as `POSCAR-1`, `test_incar`, `test_potcar`, `input.inp`, `INPUT`, and `Na_ONCV_PBE-1.0.upf`.

Running an example from the repository root with a normal command such as:

```bash
python examples/vasp/preprunvasp.py
```

makes the script look for `test_incar` and `POSCAR-1` in the repository root, even though those files live under `examples/vasp/`. The same pattern exists in the CP2K and ABACUS examples.

Expected behavior:
Each example should resolve local inputs from `Path(__file__).resolve().parent`, so the examples work regardless of the caller's current working directory.

Start with examples/vasp/preprunvasp.py, examples/cp2k/prepruncp2k.py, and examples/abacus/preprunvasp.py at the referenced input-handling sections. Run python examples/vasp/preprunvasp.py from the repository root, then repeat from another working directory, and check the corresponding CP2K and ABACUS examples. Done means every bundled input is found relative to its script directory rather than the caller's current directory.

Written by the indexing model from the issue text.