Read a prompt file from the project config dir (honouring identity profiles), falling back
to a packaged default unless noDefaultPrompts is set. Downstream packages owning their own
prompt files (e.g. the assistant's PR discovery prompt) pass defaultPromptDir pointing at
their package root; when omitted, the default is read from the core package.
Read a prompt file from the project config dir (honouring identity profiles), falling back to a packaged default unless
noDefaultPromptsis set. Downstream packages owning their own prompt files (e.g. the assistant's PR discovery prompt) passdefaultPromptDirpointing at their package root; when omitted, the default is read from the core package.