Adds shebang_templates configuration for overriding shebangs.

[zooba]

Fixes #307
Fixes #348

[zooba]

Basically just waiting on more tests, but I also haven't tested thoroughly yet so I'm sure it'll break in places. Figured I'd put it out in the open now though - I'm hoping to get it done quickly and put out a beta with it (and the other recent changes).

[Copilot]

Pull request overview

Adds configurable shebang_templates support so users can override/route non-standard (and POSIX-style) shebangs via pymanager.json, improving compatibility with legacy scripts and cross-platform shebang conventions.

Changes:

• Introduces shebang_templates config schema/defaults and wires it into shebang parsing.
• Adds _replace_templates() and integrates it into _parse_shebang() to rewrite or directly resolve template-based shebangs.
• Expands tests and test config stubs to cover the new config field and parsing paths.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
tests/test_scriptutils.py	Adds tests for shebang template replacement and _parse_shebang integration.
tests/test_install_command.py	Updates test command stub to include shebang_templates.
tests/conftest.py	Updates fake config to include shebang_templates and aligns get_install_to_run signature with production.
src/pymanager.json	Adds default shebang_templates mappings.
src/manage/scriptutils.py	Implements template replacement and integrates it into shebang parsing/warnings.
src/manage/commands.py	Adds config schema + BaseCommand default for shebang_templates; adjusts default source_settings.

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