Create a template for compilation database preparation

Right now, projects duplicate scripts to prepare compile_commands.json, which creates unmaintainable “field ware.”

Proposal

• Provide a recommended CI template job for common, easy-to-standardize steps (e.g. generate CDB, inject basic includes/flags, split for NJOBS).
• Leave hard/complex cases out of the template — these should be explicit in project configs, not hidden in the template.
• Make the template configurable via simple variables.

Benefits

• Reduce boilerplate and copy-paste.
• Keep complex logic visible and customisable.
• Make clangsa setup easier but still flexible.