Add session continuation symlink fix and /resume command

Fix session detection:
- Add save_session_continuation() calls at all session exit points
- Sessions now properly create .g3/session symlink for resume detection
- Fixes issue where g3 wasn't offering to resume previous sessions

Add /resume command:
- New list_sessions_for_directory() to scan available sessions
- New switch_to_session() method to safely switch between sessions
- Shows numbered list with timestamps, context %, and TODO status
- Saves current session before switching (can be resumed later)
- Restores full context if <80% used, otherwise uses summary
- Machine mode supports /resume and /resume <number>

Documentation:
- Add /clear and /resume to CONTROL_COMMANDS.md
- Update /help output with new commands

crates/g3-core/src/lib.rs

@@ -23,7 +23,7 @@ pub mod webdriver_session;
 pub use task_result::TaskResult;
 pub use retry::{RetryConfig, RetryResult, execute_with_retry, retry_operation};
 pub use feedback_extraction::{ExtractedFeedback, FeedbackSource, FeedbackExtractionConfig, extract_coach_feedback};
-pub use session_continuation::{SessionContinuation, load_continuation, save_continuation, clear_continuation, has_valid_continuation, get_session_dir, load_context_from_session_log, find_incomplete_agent_session};
+pub use session_continuation::{SessionContinuation, load_continuation, save_continuation, clear_continuation, has_valid_continuation, get_session_dir, load_context_from_session_log, find_incomplete_agent_session, list_sessions_for_directory, format_session_time};
 
 // Re-export context window types
 pub use context_window::{ContextWindow, ThinScope};
@@ -1528,6 +1528,42 @@ impl<W: UiWriter> Agent<W> {
         Ok(false)
     }
 
+    /// Switch to a different session, saving the current one first.
+    /// This discards the current in-memory state and loads the new session.
+    pub fn switch_to_session(
+        &mut self,
+        continuation: &crate::session_continuation::SessionContinuation,
+    ) -> Result<bool> {
+        // Save current session first (so it can be resumed later)
+        self.save_session_continuation(None);
+        
+        // Reset session-specific metrics
+        self.thinning_events.clear();
+        self.compaction_events.clear();
+        self.first_token_times.clear();
+        self.tool_call_metrics.clear();
+        self.tool_call_count = 0;
+        self.pending_90_compaction = false;
+        
+        // Update session ID to the new session
+        self.session_id = Some(continuation.session_id.clone());
+        
+        // Update agent mode info from continuation
+        self.is_agent_mode = continuation.is_agent_mode;
+        self.agent_name = continuation.agent_name.clone();
+        
+        // Load TODO content from the new session if available
+        if let Some(ref todo) = continuation.todo_snapshot {
+            // Use blocking write since we're in a sync context
+            if let Ok(mut guard) = self.todo_content.try_write() {
+                *guard = todo.clone();
+            }
+        }
+        
+        // Restore context from the continuation
+        self.restore_from_continuation(continuation)
+    }
+
     async fn stream_completion(
         &mut self,
         request: CompletionRequest,