Add JSON config file support (XDG-respecting)

End users can now set defaults in a config file at:

  ${XDG_CONFIG_HOME:-$HOME/.config}/move/config.json

CLI flags continue to win when both are set:

  CLI flags  >  config file  >  built-in defaults

Schema mirrors the CLI flag names and units. Loader is strict: unknown
keys, wrong types, and non-positive numerics are rejected with a clear
message naming the file and key, and the process exits 2.

Changes:

- New src/configFile.ts: existence-aware loader + strict schema
  validation. Throws CliError; the entry point converts those to exit 2.
- src/config.ts: ConfigOverrides gains 'verbose'; resolveConfig takes
  both file and CLI override layers; new defaultConfigPath() honors
  XDG_CONFIG_HOME; new resolveVerbose() layers verbose with the
  presence-only-CLI semantics documented.
- src/cli.ts: -C/--config <path> flag. printHelp prints the default
  config path and the precedence rule.
- src/move.ts: loads the config file (default XDG path or --config),
  passes both override layers into resolveConfig, resolves verbose
  separately, exits 2 on any validation failure.
- README: new Configuration section with path, precedence, example,
  validation rules, and the verbose CLI-can't-turn-off limitation.
  Files table gains src/configFile.ts.

src/move.ts

@@ -4,18 +4,19 @@
  * -------
  * Entry point for the `move` CLI.
  *
- * Thin shim that ties the three logic modules together:
- *   - `cli.ts`     parses and validates `process.argv`.
- *   - `config.ts`  holds the default tunables and the `resolveConfig` overlay.
- *   - `keeper.ts`  owns the synthetic-activity sweep and idle-watch loop.
+ * Thin shim that ties the four logic modules together:
+ *   - `cli.ts`         parses and validates `process.argv`.
+ *   - `configFile.ts`  loads and validates the JSON config file.
+ *   - `config.ts`      holds defaults and the layered `resolveConfig` overlay.
+ *   - `keeper.ts`      owns the synthetic-activity sweep and idle-watch loop.
  *
  * Order of operations:
- *   1. Parse CLI args. Bad input -> stderr message + short usage hint, exit 2.
- *   2. `--help` / `--version` short-circuit before any mouse work happens.
- *   3. Resolve CLI overrides on top of `DEFAULT_CONFIG` and hand the result
- *      (plus the verbose flag) to `runKeeper`.
- *   4. Any unhandled rejection from `runKeeper` is logged and exits with
- *      code 1 so it's catchable by shells / supervisors.
+ *   1. Parse CLI args. Bad input -> stderr + usage hint, exit 2.
+ *   2. `--help` / `--version` short-circuit before any I/O or mouse work.
+ *   3. Load + validate the config file (default XDG path, or `--config
+ *      <path>` if supplied). Validation failures share the exit-2 path.
+ *   4. Resolve config (CLI > file > DEFAULT_CONFIG) and the verbose flag.
+ *   5. Run keeper. Any unhandled rejection exits 1.
  *
  * Runtime: Bun (uses `@nut-tree-fork/nut-js` via `keeper.ts`). The shebang
  * above lets this file run as a real CLI once linked via `bun link`.
@@ -23,16 +24,26 @@
 
 import { parseCliArgs, printHelp, VERSION } from "./cli.ts";
 import type { ParsedCliArgs } from "./cli.ts";
-import { resolveConfig } from "./config.ts";
+import { resolveConfig, resolveVerbose } from "./config.ts";
+import type { ConfigOverrides } from "./config.ts";
+import { loadConfigFile } from "./configFile.ts";
 import { runKeeper } from "./keeper.ts";
 
+/**
+ * Print a CLI-style error and exit 2. Returns `never` so callers can use
+ * it without TypeScript flagging "variable might be undefined" downstream.
+ */
+function failUser(err: unknown): never {
+    const msg: string = err instanceof Error ? err.message : String(err);
+    process.stderr.write(`move: ${msg}\nTry 'move --help' for more information.\n`);
+    process.exit(2);
+}
+
 let cliArgs: ParsedCliArgs;
 try {
     cliArgs = parseCliArgs();
 } catch (err: unknown) {
-    const msg: string = err instanceof Error ? err.message : String(err);
-    process.stderr.write(`move: ${msg}\nTry 'move --help' for more information.\n`);
-    process.exit(2);
+    failUser(err);
 }
 
 if (cliArgs.help) {
@@ -44,14 +55,25 @@ if (cliArgs.version) {
     process.exit(0);
 }
 
-const config = resolveConfig({
-    moveInterval: cliArgs.moveInterval,
-    checkInterval: cliArgs.checkInterval,
-    stepDelay: cliArgs.stepDelay,
-    stepCount: cliArgs.stepCount,
-});
+let fileOverrides: ConfigOverrides | null;
+try {
+    fileOverrides = loadConfigFile(cliArgs.config);
+} catch (err: unknown) {
+    failUser(err);
+}
 
-runKeeper(config, cliArgs.verbose).catch((err: unknown): void => {
+const cliOverrides: ConfigOverrides = {
+    moveInterval:  cliArgs.moveInterval,
+    checkInterval: cliArgs.checkInterval,
+    stepDelay:     cliArgs.stepDelay,
+    stepCount:     cliArgs.stepCount,
+    verbose:       cliArgs.verbose,
+};
+
+const config = resolveConfig(fileOverrides, cliOverrides);
+const verbose: boolean = resolveVerbose(fileOverrides, cliOverrides);
+
+runKeeper(config, verbose).catch((err: unknown): void => {
     console.error("Error:", err);
     process.exit(1);
 });