refactor(src): narrow exception types in Phase 5 batch (8 sites across 5 files)

Migrates the 8 try/except sites in UI + theme + tooling files
by narrowing the exception types from broad 'except Exception' to
specific stdlib/domain exceptions.

Files and sites:
1. src/command_palette.py:120 (1 site) - command.action callback
   except Exception -> except (AttributeError, TypeError, ValueError, OSError)
2. src/commands.py:116 (1 site) - generate_md
   except Exception -> except (OSError, ValueError, TypeError)
3. src/commands.py:147 (1 site) - save_all
   except Exception -> except (OSError, ValueError)
4. src/commands.py:271 (1 site) - reset_layout
   except Exception -> except OSError
5. src/diff_viewer.py:167 (1 site) - apply_patch
   except Exception -> except (OSError, ValueError, IndexError)
6. src/external_editor.py:82 (1 site) - powershell reg lookup
   except Exception -> except (OSError, subprocess.SubprocessError,
                               subprocess.TimeoutExpired)
7. src/markdown_helper.py:123 (1 site) - open link
   except Exception -> except (OSError, ValueError)
8. src/markdown_helper.py:200 (1 site) - render_table fallback
   except Exception -> except (TypeError, AttributeError, ValueError, IndexError)

Also updates tests/test_command_palette_sim.py to use TypeError
(caught by the narrowing) instead of RuntimeError (not caught).

Decisions:
- theme_2.py:282 already narrow (ImportError, AttributeError); no change
- theme_models.py:166 is RAISE (not except); keep as-is (documented)
- external_editor.py:47, 56 already narrow (FileNotFoundError); no change

Tests verified:
- tests/test_command_palette.py (13 tests) PASS
- tests/test_command_palette_sim.py (7 tests) PASS
- tests/test_diff_viewer.py (10 tests) PASS
- tests/test_external_editor.py (16 tests) PASS
- tests/test_external_editor_gui.py (5 tests) PASS
- tests/test_markdown_helper_* (16 tests) PASS

src/command_palette.py

@@ -117,7 +117,7 @@ def _execute(app: Any, command: Command) -> None:
   return
  try:
   command.action(app)
- except Exception as e:
+ except (AttributeError, TypeError, ValueError, OSError) as e:
   print(f"[CommandPalette] Action {command.id} raised: {e}")
  _close_palette(app)
 

src/commands.py

@@ -113,7 +113,7 @@ def generate_md_only(app: "App") -> None:
    app.last_md_path = path
    if hasattr(app, "ai_status"):
     app.ai_status = f"md written: {path.name}"
-  except Exception as e:
+  except (OSError, ValueError, TypeError) as e:
    if hasattr(app, "ai_status"):
     app.ai_status = f"error: {e}"
 
@@ -144,7 +144,7 @@ def save_all(app: "App") -> None:
  if hasattr(app, "config"):
   try:
    app.save_config()
-  except Exception as e:
+  except (OSError, ValueError) as e:
    if hasattr(app, "ai_status"):
     app.ai_status = f"save error: {e}"
 
@@ -268,7 +268,7 @@ def reset_layout(app: "App") -> None:
    if os.path.exists(p):
     os.remove(p)
     if hasattr(app, "ai_status"): app.ai_status = f"layout reset: removed {p}"
- except Exception as e:
+ except OSError as e:
   if hasattr(app, "ai_status"): app.ai_status = f"layout reset partial: {e}"
 
 

src/diff_viewer.py

@@ -164,7 +164,7 @@ def apply_patch_to_file(patch_text: str, base_dir: str = ".") -> Tuple[bool, str
     f.writelines(new_lines)
    
    results.append(f"Patched: {file_path}")
-  except Exception as e:
+  except (OSError, ValueError, IndexError) as e:
    return False, f"Error patching {file_path}: {e}"
  
  return True, "\n".join(results)

src/external_editor.py

@@ -79,7 +79,7 @@ def _find_vscode_in_registry() -> Optional[str]:
      exe_path = line.strip() + "\\Code.exe"
      if os.path.exists(exe_path):
       paths.append(exe_path)
-  except Exception:
+  except (OSError, subprocess.SubprocessError, subprocess.TimeoutExpired):
    pass
  if paths:
   return paths[0]

src/markdown_helper.py

@@ -120,7 +120,7 @@ class MarkdownRenderer:
       webbrowser.open(str(p.absolute()))
     else:
      print(f"Link target does not exist: {url}")
-   except Exception as e:
+   except (OSError, ValueError) as e:
     print(f"Error opening link {url}: {e}")
 
  def render(self, text: str, context_id: str = "default") -> None:
@@ -197,7 +197,7 @@ class MarkdownRenderer:
     block = blocks[table_at_line[i]]
     try:
      render_table(block)
-    except Exception as e:
+    except (TypeError, AttributeError, ValueError, IndexError) as e:
      # Fallback: if table rendering fails, just append lines to md_buf
      for line_idx in range(block.span[0], block.span[1]):
       md_buf.append(lines[line_idx])

tests/test_command_palette_sim.py

@@ -146,7 +146,7 @@ def test_execute_runs_command_and_closes() -> None:
   id="test_bad",
   title="Test Bad",
   category="test",
-  action=lambda app: (_ for _ in ()).throw(RuntimeError("boom")),
+  action=lambda app: (_ for _ in ()).throw(TypeError("boom")),
  )
  # Should NOT raise
  _execute(bad_app, bad_command)