Use standard_template_library/ as a subtree from the plainlang_examples repository

[NejcS]

Summary

The standard_template_library/ is now a git subtree from the plainlang_examples repo. This PR does two things on top of that:

1. Converts the directory into a git subtree — It embeds the git-subtree-split metadata in the commit history using git subtree. This is what enables future syncing with git subtree pull when plainlang-examples updates the templates. Note that this needs to be done manually anytime the templates change in that repository.

2. Fixes plain2code.py — replaces the importlib.resources path with a plain string, so the subtree directory is resolved correctly relative to plain2code.py at runtime.

The file diff appears empty because the standard template files' contents are unchanged — only the git history wiring is new.

Also fixes pre-existing date-sensitive test failures in test_cli_output.py (hardcoded 2026-06-01 dates moved to 2028-12-01).

Test plan

• --dry-run passes for hello-world-python.plain, hello-world-golang.plain, and hello-world-react.plain (all import from the standard template library)
• All 149 unit tests pass (pytest tests/)
• test by doing a build and run the local build on a project that imports things from the library
• git subtree pull works after a future update to plainlang-examples

Closes https://github.com/Codeplain-ai/next-microsoft/issues/33

[NejcS]

Matching PR in the plainlang-examples repo: Codeplain-ai/plainlang-examples#19

@zanjonke I'd like to test if this will work for the shipped project like what clients will install from PyPI. Any suggestions how to test that locally?