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/paths.rs

@@ -1,8 +1,8 @@
 //! Path utilities for G3 session and workspace management.
 //!
 //! This module centralizes all path-related logic for:
-//! - TODO file location
-//! - Logs directory
+//! - TODO file location  
+//! - Error logs directory
 //! - Session directories and files
 //! - Thinned content storage
 
@@ -33,22 +33,29 @@ pub fn get_session_todo_path(session_id: &str) -> PathBuf {
     get_session_logs_dir(session_id).join("todo.g3.md")
 }
 
-/// Get the path to the logs directory.
+/// Get the path to the errors directory.
 ///
-/// Checks for G3_WORKSPACE_PATH environment variable first (used by planning mode),
-/// then falls back to "logs" in the current directory.
-pub fn get_logs_dir() -> PathBuf {
-    if let Ok(workspace_path) = std::env::var(G3_WORKSPACE_PATH_ENV) {
-        PathBuf::from(workspace_path).join("logs")
-    } else {
-        std::env::current_dir().unwrap_or_default().join("logs")
-    }
+/// Returns `.g3/errors/` in the workspace or current directory.
+pub fn get_errors_dir() -> PathBuf {
+    get_g3_dir().join("errors")
 }
 
-/// Public accessor for the logs directory path (for use by submodules).
-/// Alias for `get_logs_dir()` for backward compatibility.
-pub fn logs_dir() -> PathBuf {
-    get_logs_dir()
+/// Get the path to the background processes directory.
+///
+/// Returns `.g3/background_processes/` in the workspace or current directory.
+pub fn get_background_processes_dir() -> PathBuf {
+    get_g3_dir().join("background_processes")
+}
+
+/// Get the path to the discovery logs directory (for planner mode).
+///
+/// Returns `.g3/discovery/` in the workspace or current directory.
+pub fn get_discovery_dir() -> PathBuf {
+    if let Ok(workspace_path) = std::env::var(G3_WORKSPACE_PATH_ENV) {
+        PathBuf::from(workspace_path).join(".g3").join("discovery")
+    } else {
+        get_g3_dir().join("discovery")
+    }
 }
 
 /// Get the base .g3 directory path.