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.

README.md

@@ -95,17 +95,78 @@ out-of-bounds events.
 Invalid input (unknown flag, missing value, non-positive number) prints an
 error to `stderr` and exits with code `2`.
 
+## Configuration
+
+`move` reads an optional JSON config file at:
+
+```
+${XDG_CONFIG_HOME:-$HOME/.config}/move/config.json
+```
+
+If the file is missing, defaults are used (no config file is required).
+Pass `-C` / `--config <path>` to point at a different file; in that mode
+the file must exist.
+
+### Precedence
+
+```
+CLI flags  >  config file  >  built-in defaults
+```
+
+CLI flags always win. The config file fills in any flag the user didn't
+pass on the command line. Built-in defaults fill in anything the file
+doesn't set.
+
+### Example
+
+```jsonc
+{
+  "moveInterval":  240,
+  "checkInterval": 10,
+  "stepDelay":     50,
+  "stepCount":     250,
+  "verbose":       false
+}
+```
+
+All keys are optional; supply only the ones you want to override. Keys
+and units mirror the CLI flags exactly: `moveInterval` and
+`checkInterval` are seconds, `stepDelay` is milliseconds, `stepCount` is
+pixels, `verbose` is a boolean.
+
+### Validation
+
+The loader is strict:
+
+- Root must be a JSON object.
+- Unknown keys are rejected (catches typos like `"movInterval"`).
+- Numeric values must be finite and strictly positive.
+- `verbose` must be a boolean.
+
+Any validation failure prints a message naming the file and the offending
+key to `stderr` and exits `2`.
+
+### Known limitation: `verbose` can be turned on but not off from the CLI
+
+`--verbose` is a presence-only flag (there is no `--no-verbose`). If the
+config file sets `"verbose": true`, the CLI cannot force quiet mode in
+that invocation. Workarounds: edit the file, or point at a different
+file with `--config`.
+
 ## How it works
 
-The source lives under `src/`, split into an entry point plus three logic
+The source lives under `src/`, split into an entry point plus four logic
 modules:
 
 - `src/move.ts` is a thin entry point: parses args, dispatches `--help` /
-  `--version`, resolves the runtime config, and calls
-  `runKeeper(config, verbose)`.
+  `--version`, loads the config file, resolves the layered runtime
+  config, and calls `runKeeper(config, verbose)`.
 - `src/cli.ts` owns argument parsing, validation, and help/version output.
-- `src/config.ts` exports the `Config` type, `DEFAULT_CONFIG`, and the
-  `resolveConfig` overlay function.
+- `src/configFile.ts` owns optional JSON config-file loading + strict
+  schema validation.
+- `src/config.ts` exports the `Config` type, `DEFAULT_CONFIG`,
+  `defaultConfigPath`, and the layered `resolveConfig` / `resolveVerbose`
+  overlay functions.
 - `src/keeper.ts` owns the synthetic-activity sweep and the idle-watch loop.
 
 Defaults live in `src/config.ts` as `DEFAULT_CONFIG`:
@@ -190,7 +251,8 @@ move --help
 | `scripts/dev-setup.sh` | Contributor bootstrap (verify Bun + `bun install`).                        |
 | `src/move.ts`       | CLI entry point: parses args, dispatches help/version, starts the loop.       |
 | `src/cli.ts`        | Argument parsing, validation, and help/version output.                        |
-| `src/config.ts`     | `Config` type, `DEFAULT_CONFIG`, and `resolveConfig` overlay.                 |
+| `src/config.ts`     | `Config` type, `DEFAULT_CONFIG`, `defaultConfigPath`, and the layered `resolveConfig` / `resolveVerbose` overlay. |
+| `src/configFile.ts` | Optional JSON config-file loader with strict schema validation.               |
 | `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).          |