refactor: DRY cleanup — extract shared helpers, remove duplication across tools, LSP, overlays, and runtime

Eliminate ~500 lines of duplicate code across 31 files by extracting
shared functions, helpers, and consolidating repeated patterns.

Highlights:
- Toast helpers (toast_info/success/warning/error) on AppStateRest
- push_event() helper for turn-event queue (19 callers consolidated)
- log_write_edit_tool() shared fn (turn.rs + engine.rs ~50 lines saved)
- resolve_api_key() shared fn (spawn.rs + provider.rs)
- LSP call_positional() helper on LspClient
- lsp_cursor_params() shared schema for 4 tool files
-overlay_block() helper for consistent overlay title/border styling
- cycle_selected_index(), path_not_found/a_directory() helpers
- mark_dirty(), save_settings() on AppStateRest
- Remove redundant Err(e) => Err(e) arms in LSP tools
- Consolidate generate_workspace_tree (turn.rs → workspace.rs)
- Simplify background-review wrapper args in auto/mod.rs

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/zesdex-backend/src/app/mode/effort.rs

@@ -47,9 +47,6 @@ pub fn cycle_effort(state: &mut AppStateRest) {
     let current = current_effort(state);
     state.misc.effort_level = (current + 1) % EFFORT_LEVELS.len();
     let label = current_effort_str(state);
-    state.push_toast(crate::app::state::types::Toast::new(
-        crate::app::state::types::ToastKind::Info,
-        format!("Effort: {label}"),
-    ));
+    state.toast_info(format!("Effort: {label}"));
     state.dirty = true;
 }