refactor(g3-core): use StreamingState and IterationState structs in stream_completion_with_tools

Consolidate scattered state variables in the 834-line stream_completion_with_tools()
function to use the existing StreamingState and IterationState structs from
streaming.rs. This eliminates code-path aliasing where state was tracked in
multiple places and makes the streaming loop easier to reason about.

Changes:
- Add assistant_message_added field to StreamingState
- Add stream_stop_reason field to IterationState
- Replace 8 inline state variables with StreamingState::new()
- Replace 7 iteration-local variables with IterationState::new()
- All 585 workspace tests pass

This is a pure refactor with no behavior changes. The state structs were already
defined in streaming.rs but not used in the main streaming loop.

Agent: fowler

analysis/memory.md

@@ -1,5 +1,5 @@
 # Project Memory
-> Updated: 2026-01-20T08:53:25Z | Size: 16.3k chars
+> Updated: 2026-01-20T09:01:08Z | Size: 16.7k chars
 
 ### Remember Tool Wiring
 - `crates/g3-core/src/tools/memory.rs` [0..5000] - `execute_remember()`, `get_memory_path()`, `merge_memory()`
@@ -295,4 +295,11 @@ Shared display functions for interactive and agent modes.
   - `print_workspace_path()` [20..29] - prints formatted workspace path
   - `LoadedContent` [32..39] - tracks loaded project files (README, AGENTS.md, Memory, include prompt)
   - `print_loaded_status()` [87..103] - prints "✓ README  ✓ AGENTS.md" status line
-  - `print_project_heading()` [106..114] - prints project name from README
+  - `print_project_heading()` [106..114] - prints project name from README
+
+### Interactive Commands Module
+Handles `/` commands in interactive mode (extracted from interactive.rs).
+
+- `crates/g3-cli/src/commands.rs`
+  - `handle_command()` [17..320] - dispatches `/help`, `/compact`, `/thinnify`, `/skinnify`, `/fragments`, `/rehydrate`, `/run`, `/dump`, `/clear`, `/readme`, `/stats`, `/resume`
+  - Returns `Result<bool>` - true if command handled and loop should continue