fix(dsl-engine): de-noise python pipeline load errors

[markovejnovic]

Problem

When a user's .hm/*.py raises at import time — a removed kwarg (e.g. default_image, see #147/#148), a typo, a bad import — the render leaks the raw CPython traceback, including the harness <string> frame and the <frozen importlib._bootstrap> machinery the user can't act on:

render failed (exit 1): rendering pipeline "ci": rendering pipeline via python3: python3 exited with code 1:
Traceback (most recent call last):
  File "<string>", line 11, in <module>
  File "<frozen importlib._bootstrap_external>", line 759, in exec_module
  File "<frozen importlib._bootstrap>", line 491, in _call_with_frames_removed
  File "/tmp/co/.hm/ci.py", line 66, in <module>
    @hm.pipeline(
  ...
TypeError: pipeline() got an unexpected keyword argument 'default_image'

This is the failure prod has been surfacing on builds, and it violates the error-message doctrine (point precisely, state what was observed, fail fast — no noise).

Fix

Factor the three render scripts (list / registry / render) onto a shared bootstrap (_load_pipelines) that wraps exec_module and, on failure, reports only what's actionable — the offending file, the exception, and the frames inside the user's own pipeline file — dropping the harness and importlib frames:

error: failed to load pipeline file '.hm/ci.py'
  TypeError: pipeline() got an unexpected keyword argument 'default_image'
  at .hm/ci.py:66 in <module>

Frame matching tolerates relative/absolute path differences so the precise line always shows.

This is the general fix; #148 separately restores default_image as a deprecation shim so that specific kwarg keeps working during the grace period. Together: old pipelines keep rendering, and any future load-time error reads cleanly.

Tests

tests/python_engine_test.rs::python_load_error_is_denoised — a pipeline file that raises at import yields an error that names the file, surfaces the exception, points at the failing line (ci.py:3), and contains no _bootstrap / exec_module frames.

Crate: all tests pass; clippy -D warnings clean.