Part of the domain-reorg adoption (build plan step C1): fork content,
upstream layout. Moves INSTALLATION.md/CUSTOMIZATION.md under docs/,
static/ under assets/, and default_prompt.md under agent/resources/
(packaged via agent/resources/__init__.py), then switches prompt.py's
loader to importlib.resources with an explicit DEFAULT_PROMPT_PATH
override, matching upstream's hunk. README and CUSTOMIZATION.md links
updated for the new paths; wheel build verified to still ship
agent/resources/default_prompt.md.
