Remove automatic README loading from context window

README.md is no longer auto-loaded into the LLM context at startup.
This saves ~4,600 tokens per session while AGENTS.md and memory.md
still provide all critical information for code tasks.

Changes:
- Delete read_project_readme() function
- Remove readme_content parameter from combine_project_content()
- Rename extract_readme_heading() -> extract_project_heading()
- Rename Agent constructors: *_with_readme_* -> *_with_project_context_*
- Update context preservation to only check for Agent Configuration
- Remove has_readme field from LoadedContent
- Update all tests to use new markers and function names

The LLM can still read README.md on-demand via read_file when needed.

crates/g3-cli/src/autonomous.rs

@@ -501,7 +501,7 @@ async fn execute_coach_turn(
     let ui_writer = ConsoleUiWriter::new();
     ui_writer.set_workspace_path(project.workspace().to_path_buf());
     let mut coach_agent =
-        match Agent::new_autonomous_with_readme_and_quiet(coach_config, ui_writer, None, quiet)
+        match Agent::new_autonomous_with_project_context_and_quiet(coach_config, ui_writer, None, quiet)
             .await
         {
             Ok(a) => a,