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.

src/cli.ts

@@ -9,6 +9,9 @@
  *
  *   -h, --help              Prints `printHelp()` to stdout; entry exits 0.
  *   -v, --version           Prints `move <VERSION>` to stdout; entry exits 0.
+ *   -e, --edit              Open the active config file in `$EDITOR`.
+ *                           Refuses if the file doesn't exist; refuses if
+ *                           `$EDITOR` is unset.
  *   -C, --config <path>     Override the default config-file path.
  *   -m, --move-interval     Idle time (seconds) before a sweep fires.
  *   -c, --check-interval    Cursor poll cadence (seconds).
@@ -37,6 +40,7 @@ import { CliError } from "./errors.ts";
 export interface ParsedCliArgs {
     help: boolean;
     version: boolean;
+    edit: boolean;
     config: string | undefined;
     moveInterval: number | undefined;   // seconds
     checkInterval: number | undefined;  // seconds
@@ -78,6 +82,7 @@ export function parseCliArgs(): ParsedCliArgs {
             options: {
                 help:             { type: "boolean", short: "h" },
                 version:          { type: "boolean", short: "v" },
+                edit:             { type: "boolean", short: "e" },
                 config:           { type: "string",  short: "C" },
                 "move-interval":  { type: "string",  short: "m" },
                 "check-interval": { type: "string",  short: "c" },
@@ -99,6 +104,7 @@ export function parseCliArgs(): ParsedCliArgs {
     return {
         help:          Boolean(values.help),
         version:       Boolean(values.version),
+        edit:          Boolean(values.edit),
         config:        values.config as string | undefined,
         moveInterval:  parsePositiveNumber("move-interval",  values["move-interval"]  as string | undefined),
         checkInterval: parsePositiveNumber("check-interval", values["check-interval"] as string | undefined),
@@ -140,6 +146,7 @@ nudging the mouse cursor after a configurable idle period.
 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: ${cfgPath}
   -m, --move-interval <seconds>   Idle time before a sweep fires. Default: ${moveDefaultSec}.