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-planner/src/llm.rs

@@ -357,7 +357,7 @@ pub async fn call_refinement_llm_with_tools(
     project.enter_workspace()?;
 
     // Create agent - not autonomous mode, just regular agent with tools
-    let mut agent = Agent::new_with_readme_and_quiet(
+    let mut agent = Agent::new_with_project_context_and_quiet(
         planner_config,
         ui_writer,
         Some(system_prompt),