Centralize g3 status message formatting

Extract a new g3_status module in g3-cli that provides consistent formatting
for all 'g3:' prefixed system status messages.

Key changes:
- Add G3Status struct with methods for progress, done, failed, error, etc.
- Add Status enum with Done, Failed, Error, Resolved, Insufficient, NoChanges
- Add ThinResult struct in g3-core for semantic thinning data
- Update UiWriter trait with print_thin_result() method
- Refactor context thinning to return ThinResult instead of formatted strings
- Update all callers to use the new centralized formatting
- Session resume/decline messages now use G3Status
- Compaction status messages now use G3Status

This maintains clean separation of concerns: g3-core emits semantic data,
g3-cli handles all terminal formatting and colors.

crates/g3-core/tests/todo_staleness_test.rs

@@ -54,7 +54,7 @@ impl UiWriter for MockUiWriter {
     }
     fn print_g3_progress(&self, _message: &str) {}
     fn print_g3_status(&self, _message: &str, _status: &str) {}
-    fn print_context_thinning(&self, _message: &str) {}
+    fn print_thin_result(&self, _result: &g3_core::ThinResult) {}
     fn print_tool_header(&self, _tool_name: &str, _tool_args: Option<&serde_json::Value>) {}
     fn print_tool_arg(&self, _key: &str, _value: &str) {}
     fn print_tool_output_header(&self) {}