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/tool/utility/cd.rs

@@ -45,18 +45,10 @@ impl Tool for Cd {
         let path = super::super::resolve_path(&ctx.workspaces, &rel)?;
 
         if !path.exists() {
-            return Ok(format!(
-                "path '{}' does not exist (resolved to {})",
-                rel,
-                path.display()
-            ));
+            return Ok(super::path_not_found(&rel, &path));
         }
         if !path.is_dir() {
-            return Ok(format!(
-                "path '{}' is not a directory (resolved to {})",
-                rel,
-                path.display()
-            ));
+            return Ok(super::path_not_a_directory(&rel, &path));
         }
 
         let canon = path.canonicalize().unwrap_or(path);