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 documentation and add a regression test for consistent site/platform names.
• Preserve the current upstream site configuration, including fre-nctools/2026.01.

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 its output
• I ran pylint and attempted to implement some of its feedback
• No print statements; all user-facing info uses logging module

Validation

• cylc lint -v: passed.
• Focused Jinja2, site-name, PPAN handler, and Cylc tests: 28 passed.
• pylint --rcfile ./pylintrc --fail-under 9.8 ./lib/python: 9.83/10.
• Broader Jinja2 and Python suite: 33 passed, 2 failed. Both failures also reproduce on untouched upstream on macOS because they expect Linux mv error text.
• git diff --check: passed.
• Independent review confirmed site/platform consistency and preservation of current upstream configuration.

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. Local validation does not replace this end-to-end run.

[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.

[ColumbusLabs]

Friendly ping: this is rebased on current main and ready for review. Happy to make any changes, and could a maintainer trigger test_cloud_runner (fork ColumbusLabs/fre-workflows, branch codex-rename-ppan-sites) when convenient?