Refactor system prompts to eliminate duplication; upgrade embedded provider
- Refactor prompts.rs: extract shared sections (intro, TODO, workspace memory,
web research, response guidelines) used by both native and non-native prompts
- Fix typo in native prompt: "save them.." -> "save them."
- Fix non-native prompt: add missing closing braces in JSON examples,
add IMPORTANT steps section, align with native prompt quality
- Add 9 unit tests to verify both prompts contain required sections
- Upgrade llama-cpp-2 dependency and refactor embedded provider
- Update config.example.toml with embedded model examples
- Update workspace memory
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