Rename PPAN sites to reflect EPMT behavior

[ColumbusLabs]

Describe your changes

• Rename ppan_test to ppan_epmt and ppan to ppan_noepmt across site files, platform definitions, and workflow conditionals.
• Make ppan_epmt the documented/local default while retaining ppan_noepmt as the explicit opt-out.
• Update developer and portability docs and add a regression test for consistent site/platform names.

Issue ticket number and link (if applicable)

Closes #172

Checklist before requesting a review

• I ran my code
• I tried to make my code readable
• I tried to comment my code
• I wrote a new test, if applicable
• I wrote new instructions/documentation, if applicable
• I ran pytest and inspected it's output
• I ran pylint and attempted to implement some of it's feedback
• No print statements; all user-facing info uses logging module

Validation

• cylc lint -v (passes, no issues)
• pytest -q Jinja2Filters/tests lib/python/tests/test_site_names.py lib/python/tests/test_ppan_handler.py (22 passed)
• pylint --rcfile ./pylintrc --fail-under 9.8 ./lib/python (9.83/10)
• git diff --check

Manual Pipeline Run Details

Was the manual pipeline (test_cloud_runner) triggered for this PR?

• Yes
• No

Result of manual pipeline run:

Awaiting a maintainer-triggered run using fork ColumbusLabs/fre-workflows, branch codex-rename-ppan-sites, and the appropriate fre-cli branch.

[ColumbusLabs]

The PR checklist is now complete and the focused validation passes. Could a maintainer please run test_cloud_runner with fork ColumbusLabs/fre-workflows, branch codex-rename-ppan-sites, and the appropriate fre-cli branch? I will update the manual-pipeline section with the result afterward.