Allow configurable Cylc run directory

[mulroony]

Allow the Cylc run directory to be configured

Closes #7410

Summary

Allow the CYLC_RUN_DIR environment variable to override the standard $HOME/cylc-run workflow run root.

The default remains unchanged when the variable is unset.

Changes

• Use CYLC_RUN_DIR in local and remote workflow path construction.
• Automatically forward CYLC_RUN_DIR over Cylc-managed SSH commands.
• Export it in generated job scripts so clean job-submission environments retain the configured run root.
• Document the environment variable and its interaction with symlink directories.
• Preserve the precedence of explicit alternate run-directory arguments.

This differs from [install][symlink dirs]: symlink configuration moves workflow data but retains the canonical namespace under $HOME/cylc-run. CYLC_RUN_DIR moves that namespace itself for sites where home directories cannot be used for workflow runtime state.

Testing

• 128 focused path, remote-command, job-file, and global-config unit tests passed.
• 222 install, workflow-file, ID parsing, cleanup, completion, and integration tests passed.
• Manual smoke test confirmed that cylc install places a workflow under CYLC_RUN_DIR and that cylc validate finds it there.

Check list

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes.
• Does not contain off-topic changes.
• No dependency changes.
• Appropriate tests are included.
• Changelog fragment added after the PR number is assigned.
• Cylc documentation PR opened, if requested by the maintainers.

[hjoliver]

(See discussion on the associated issue #7410, on whether we need this or not).