Add -e/--edit flag: open config file in $EDITOR

New module src/editor.ts handles the flag end-to-end:

  - editorCommand(editor, path) returns the sh -c argv that lets the
    shell tokenize multi-word $EDITOR values like 'code --wait'.
    Extracted so editor.test.ts can verify construction without
    actually launching an editor.
  - editConfig(path) checks $EDITOR is set, checks the target file
    exists, spawns 'sh -c <editor> "$@" -- <path>' with stdio
    inherited, and exits with the editor's status code.

Refuses (CliError -> exit 2) when:
  - $EDITOR is unset or empty.
  - The target config file doesn't exist. (Same recovery hint as
    elsewhere: 'run move once or reinstall'.)

src/cli.ts adds the flag to the parser and printHelp(). src/move.ts
dispatches it after --version and before config-load. Resolution
mirrors the loader: --config <path> wins, else defaultConfigPath().

8 new tests in src/editor.test.ts cover the pure helper and both
refusal paths; the spawn success path is verified via manual
'EDITOR=true move -e' (would otherwise kill the test process).

README Usage block, Configuration section (new Editing subsection),
and Files table all updated to match.

README.md

@@ -84,6 +84,7 @@ Usage: move [options]
 Options:
   -h, --help                      Show this help and exit.
   -v, --version                   Print version and exit.
+  -e, --edit                      Open the config file in $EDITOR and exit.
   -C, --config <path>             Load defaults from a JSON config file.
                                   Default path: see the Configuration section.
   -m, --move-interval <seconds>   Idle time before a sweep fires. Default: 240.
@@ -155,6 +156,24 @@ and units mirror the CLI flags exactly: `moveInterval` and
 `checkInterval` are seconds, `stepDelay` is milliseconds, `stepCount` is
 pixels, `verbose` is a boolean.
 
+### Editing
+
+```sh
+move -e         # or --edit
+move --edit --config /path/to/another.json
+```
+
+Opens the active config file in `$EDITOR` (honors flags in the value,
+so `EDITOR="code --wait"` and `EDITOR=vim` both work). Refuses with
+exit `2` if:
+
+- `$EDITOR` is unset or empty.
+- The target file doesn't exist. (Run `move` once or reinstall to
+  re-seed the default file.)
+
+The editor's own exit code is propagated, so you can chain
+`move -e && move` to validate-by-running after every edit.
+
 ### Validation
 
 The loader is strict:
@@ -275,6 +294,8 @@ move --help
 | `src/cli.ts`        | Argument parsing, validation, and help/version output.                        |
 | `src/config.ts`     | `Config` type (carries every tunable, including `verbose`), `DEFAULT_CONFIG` (derived from `scripts/config.default.json`), `defaultConfigPath`, and the layered `resolveConfig` overlay. |
 | `src/configFile.ts` | Optional JSON config-file loader with strict schema validation.               |
+| `src/editor.ts`     | `move --edit`: opens the active config file in `$EDITOR`.                     |
+| `src/errors.ts`     | Shared error types (`CliError`).                                              |
 | `src/keeper.ts`     | Synthetic-activity sweep and idle-watch loop.                                 |
 | `package.json`      | Bun project manifest. Single runtime dep: `@nut-tree-fork/nut-js`.            |
 | `tsconfig.json`     | Strict TypeScript config tuned for Bun (ESNext, bundler resolution).          |