fix(config): restore default system prompt on upgrade for uncustomized configs

[quiet-node]

Summary

• Adds system_customized: bool to PromptSection to distinguish deliberate clears from old-config artifacts
• Loader now restores DEFAULT_SYSTEM_PROMPT_BASE when system_customized is false and system is empty, healing configs from before the Settings UI existed where system = "" was the compiled default
• write_field_to_disk co-writes system_customized = true whenever prompt.system is saved via Settings, so future intentional clears are respected

Why

Old configs (pre-Settings UI) were seeded with system = "" as the compiled default. After the Settings UI shipped, loading those configs sent no persona at all instead of the built-in secretary prompt. The system_customized flag is the minimal sentinel needed to tell these two cases apart without a schema version bump.

Test plan

• bun run test:all:coverage passes (837 backend tests, coverage gate clean)
• bun run validate-build passes (zero warnings, zero errors)
• Manually upgrade a config with system = "" and no system_customized key: confirm default prompt is restored on next load
• Save a blank system prompt via Settings: confirm system_customized = true is written to TOML and empty is preserved on next load