fix: correct type() misuse, unused imports, exit(), and timezone-naiv…

[Acuspeedster]

fix: correct type() misuse, unused imports, exit(), and timezone-naive
datetime

• jinja/tags.py: replace type(x is str) with isinstance(x, str).
  type(kwargs['format'] is str) always evaluated to <class 'bool'>
  (truthy), making the else branch (default format fallback)
  unreachable dead code and silently passing non-string values to
  strftime().

• catalog_merger.py: remove 7 unused list_utils imports that were
  never used (as_dict, as_filtered_list, as_list,
  delete_item_from_list, deep_set, get_item_from_list,
  none_if_empty). Only set_or_pop is actually referenced.

• cli.py: replace exit() with sys.exit(). The built-in exit() is
  intended for interactive interpreter use; sys.exit() is correct
  for application code. Added missing import sys.

• cache.py: pass tz=datetime.timezone.utc to both fromtimestamp()
  and now() so the timedelta calculation is timezone-aware and
  immune to DST transitions.

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Types of changes

<!--- What types of changes does your code introduce? Put an `x` in all the boxes that apply: -->

• Hot fix (emergency fix and release)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Documentation (change which affects the documentation site)
• Breaking change (fix or feature that would cause existing
  functionality to change)
• Release (develop -> main)

Quality assurance (all should be covered).

• My code follows the code style of this project.
• Documentation for my change is up to date?
• My PR meets testing requirements.
• All new and existing tests passed.
• All commits are signed-off.

How To Test

If using act (https://github.com/nektos/act), fill in below:

Summary

Four independent bug fixes identified during static analysis. These
changes correct logic errors, remove dead imports, ensure correct
program exit behavior, and make datetime arithmetic timezone-safe. No
functional changes beyond fixing the identified bugs.

Files changed

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File	Change
trestle/core/jinja/tags.py	type(x is str) → isinstance(x, str)
trestle/core/catalog/catalog_merger.py	Remove unused list_utils imports
trestle/cli.py	Replace exit() with sys.exit()
trestle/core/remote/cache.py	Use datetime.timezone.utc for datetime arithmetic

Testing

• Existing Jinja and cache tests pass
• ruff check clean for modified files
• Changes affect already-covered code paths

Key links:

• Sonar
  coverage

Before you merge

• Ensure it is a 'squash commit' if not a release.
• Ensure CI is currently passing
• Check sonar. If you are working for a fork a maintainer will reach
  out, if required.

[Acuspeedster]

@vikas-agarwal76
@degenaro
@butler54

[butler54]

Please fix the merge conflicts and re-request review.