refactor(g3-core): remove 3 unused Agent constructor variants

Remove dead code - constructor variants that had no callers:
- new_with_readme()
- new_autonomous_with_readme()
- new_with_quiet()

These were thin wrappers around new_with_mode_and_readme() that were
never used externally. All 5 remaining constructors have verified callers.

Results:
- lib.rs reduced from 2817 to 2797 lines (-20 lines)
- Eliminated code-path aliasing: 8 constructors → 5 constructors
- All g3-core tests pass
- Full workspace compiles cleanly

Agent: fowler

crates/g3-core/src/lib.rs

@@ -137,30 +137,10 @@ impl<W: UiWriter> Agent<W> {
         Self::new_with_mode(config, ui_writer, false, false).await
     }
 
-    pub async fn new_with_readme(
-        config: Config,
-        ui_writer: W,
-        readme_content: Option<String>,
-    ) -> Result<Self> {
-        Self::new_with_mode_and_readme(config, ui_writer, false, readme_content, false, None).await
-    }
-
-    pub async fn new_autonomous_with_readme(
-        config: Config,
-        ui_writer: W,
-        readme_content: Option<String>,
-    ) -> Result<Self> {
-        Self::new_with_mode_and_readme(config, ui_writer, true, readme_content, false, None).await
-    }
-
     pub async fn new_autonomous(config: Config, ui_writer: W) -> Result<Self> {
         Self::new_with_mode(config, ui_writer, true, false).await
     }
 
-    pub async fn new_with_quiet(config: Config, ui_writer: W, quiet: bool) -> Result<Self> {
-        Self::new_with_mode(config, ui_writer, false, quiet).await
-    }
-
     pub async fn new_with_readme_and_quiet(
         config: Config,
         ui_writer: W,