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

crates/g3-providers/Cargo.toml

@@ -27,6 +27,6 @@ nanoid = "0.4"
 serde_urlencoded = "0.7"
 tokio-util = "0.7"
 dirs = "5.0"
-llama_cpp = { version = "0.3.2", features = ["metal"] }
+llama-cpp-2 = { version = "0.1", features = ["metal"] }
 shellexpand = "3.1"
 rand = "0.8"