Remove legacy logs/ directory, consolidate all data under .g3/

This change removes the legacy logs/ directory and consolidates all
session data, error logs, and discovery files under the .g3/ directory.

New directory structure:
- .g3/sessions/<session_id>/session.json - session logs
- .g3/errors/ - error logs (was logs/errors/)
- .g3/background_processes/ - background process logs
- .g3/discovery/ - planner discovery files (was workspace/logs/)

Changes:
- paths.rs: Remove get_logs_dir()/logs_dir(), add get_errors_dir(),
  get_background_processes_dir(), get_discovery_dir()
- session.rs: Anonymous sessions now use .g3/sessions/anonymous_<ts>/
- error_handling.rs: Errors now saved to .g3/errors/
- project.rs: Remove logs_dir() and ensure_logs_dir() methods
- feedback_extraction.rs: Remove logs_dir field and fallback logic
- planner: Use .g3/ for workspace data and .g3/discovery/ for reports
- flock.rs: Look for session metrics in .g3/sessions/
- coach_feedback.rs: Remove fallback to logs/ path
- Update all tests to use new paths
- Update README.md and .gitignore

crates/g3-core/src/lib.rs

@@ -53,10 +53,11 @@ use std::time::{Duration, Instant};
 use tokio_util::sync::CancellationToken;
 use tracing::{debug, error, warn};
 
-// Re-export path utilities for backward compatibility
+// Re-export path utilities
 pub use paths::{
-    G3_WORKSPACE_PATH_ENV, ensure_session_dir, get_context_summary_file, get_g3_dir, get_logs_dir,
-    get_session_file, get_session_logs_dir, get_session_todo_path, get_thinned_dir, logs_dir,
+    G3_WORKSPACE_PATH_ENV, ensure_session_dir, get_context_summary_file, get_g3_dir,
+    get_session_file, get_session_logs_dir, get_session_todo_path, get_thinned_dir,
+    get_errors_dir, get_background_processes_dir, get_discovery_dir,
 };
 use paths::get_todo_path;
 
@@ -291,7 +292,7 @@ impl<W: UiWriter> Agent<W> {
             working_dir: None,
             background_process_manager: std::sync::Arc::new(
                 background_process::BackgroundProcessManager::new(
-                    paths::get_logs_dir().join("background_processes")
+                    paths::get_background_processes_dir()
                 )),
             pending_images: Vec::new(),
             is_agent_mode: false,