Initial commit: move CLI (milestone 2)

- src/move.ts entry point with CLI parsing, --help, --version
- src/cli.ts: parseCliArgs, printHelp, ParsedCliArgs, CliError, VERSION
- src/config.ts: Config type, DEFAULT_CONFIG, resolveConfig
- src/keeper.ts: synthetic-activity sweep + idle-watch loop
- package.json bin entry + shebang for 'bun link' global install
- install.sh: contributor bootstrap (will be repurposed; see
  DISTRIBUTION-PLAN.md for the end-user installer design)
- DISTRIBUTION-PLAN.md captures the tabled end-user distribution work

src/config.ts

@@ -0,0 +1,76 @@
+/**
+ * config.ts
+ * ---------
+ * Runtime configuration types, defaults, and the CLI->config resolver.
+ *
+ * The keeper is driven by a single `Config` object that carries the four
+ * tunables it cares about. Defaults live in `DEFAULT_CONFIG`; CLI overrides
+ * are layered on top by `resolveConfig` rather than mutating the defaults,
+ * so the defaults stay genuinely constant and the resolved config stays
+ * structurally typed.
+ *
+ * All fields are in their internal units (ms, pixels). The CLI exposes the
+ * time-valued fields in seconds for ergonomics; `resolveConfig` performs
+ * the seconds->ms conversion at the boundary so downstream code never has
+ * to think about it.
+ */
+
+/**
+ * The shape of a resolved runtime configuration. `readonly` to make
+ * accidental mutation a type error.
+ *
+ * - `moveInterval`  — how long the mouse must be idle (no real movement)
+ *                     before a synthetic sweep is triggered. Milliseconds.
+ * - `checkInterval` — cadence of the idleness poll in the main loop.
+ *                     Milliseconds.
+ * - `stepDelay`     — pause between individual synthetic mouse steps inside
+ *                     a sweep. Also the window in which the user can
+ *                     "interrupt" by moving the cursor. Milliseconds.
+ * - `stepCount`     — number of pixel-steps in a single sweep. Pixels.
+ */
+export interface Config {
+    readonly moveInterval: number;
+    readonly checkInterval: number;
+    readonly stepDelay: number;
+    readonly stepCount: number;
+}
+
+/**
+ * Built-in defaults used when the user did not supply an explicit CLI
+ * override for the corresponding flag.
+ */
+export const DEFAULT_CONFIG: Config = {
+    moveInterval: 4 * 60 * 1000, // 4 minutes in milliseconds
+    checkInterval: 10 * 1000,    // Check every 10 seconds
+    stepDelay: 50,               // ms between mouse steps
+    stepCount: 250,              // pixels to move per sweep
+};
+
+/**
+ * Subset of `ParsedCliArgs` that `resolveConfig` actually consumes. Declared
+ * locally instead of importing from `cli.ts` to keep the dependency arrow
+ * pointing one way (cli -> config), which lets `config.ts` stay a leaf
+ * module with no internal imports.
+ */
+export interface ConfigOverrides {
+    readonly moveInterval: number | undefined;   // seconds (CLI units)
+    readonly checkInterval: number | undefined;  // seconds (CLI units)
+    readonly stepDelay: number | undefined;      // milliseconds
+    readonly stepCount: number | undefined;      // pixels
+}
+
+/**
+ * Overlay user-supplied CLI values on top of `DEFAULT_CONFIG` and return a
+ * resolved `Config`. Time-valued CLI inputs (move/check interval) are in
+ * seconds; this is where they're converted to milliseconds for internal use.
+ *
+ * Any field left `undefined` in the overrides falls back to the default.
+ */
+export function resolveConfig(overrides: ConfigOverrides): Config {
+    return {
+        moveInterval: overrides.moveInterval !== undefined ? overrides.moveInterval * 1000 : DEFAULT_CONFIG.moveInterval,
+        checkInterval: overrides.checkInterval !== undefined ? overrides.checkInterval * 1000 : DEFAULT_CONFIG.checkInterval,
+        stepDelay: overrides.stepDelay ?? DEFAULT_CONFIG.stepDelay,
+        stepCount: overrides.stepCount ?? DEFAULT_CONFIG.stepCount,
+    };
+}