feat(provider): move system prompt block to file and add pre-save quality gate

feat(provider): move system prompt block to file and add pre-save quality gate

Move the inline string concatenation from system_prompt_block() into
src/retinue/system_prompt_block.txt so the Python source is readable
and the discipline text is editable without code changes.

Add a pre-save quality gate to the discipline text:
- search existing memories before adding,
- fresh-session test before saving,
- quote file paths that contain spaces,
- name all referents,
- avoid bare pronouns,
- one fact per memory.

Bump plugin version to 1.1.0.