New installs get a cleaner structure:
cache/images/ (was image_cache/)
cache/audio/ (was audio_cache/)
cache/documents/ (was document_cache/)
cache/screenshots/ (was browser_screenshots/)
platforms/whatsapp/session/ (was whatsapp/session/)
platforms/matrix/store/ (was matrix/store/)
platforms/pairing/ (was pairing/)
Existing installs are unaffected -- get_hermes_dir() checks for the
old path first and uses it if present. No migration needed.
Adds get_hermes_dir(new_subpath, old_name) helper to hermes_constants.py
for reuse by any future subsystem.
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