Move docs/ package mirror into document-navigator plugin

Relocate the ~6k-file docs tree from the repo root into
plugins/document-navigator/docs/ so the plugin is self-contained and
portable. docs/superpowers/ (specs and plans) stays at the repo root.

Moved via cp+rm rather than git mv because directory rename() fails with
EPERM on this WSL /mnt DrvFs mount; git rename-detection pairs the
identical-content files at commit time.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

plugins/document-navigator/docs/foundation/timing/datetime/src/business-days/business-days.md

@@ -0,0 +1,100 @@
+# business-days
+
+<!-- begin:auto-docs -->
+_(no submodules)_
+<!-- end:auto-docs -->
+## Purpose
+
+The `business-days` module provides a complete suite of working-day arithmetic functions for `@suaveplan/datetime`. It models "business day" as any day that is neither in the caller-supplied `weekendDays` set (default: Sunday and Saturday) nor in the caller-supplied `holidays` list. This allows the same API to serve different jurisdictions and corporate calendars without baking in any cultural assumptions.
+
+All functions accept the polymorphic `DateInput` type and delegate to `toDate` for coercion, so callers can pass ISO strings, epoch milliseconds, or `Date` objects interchangeably.
+
+## Features
+
+- `isBusinessDay` — predicate for whether a given date is a working day
+- `nextBusinessDay` / `previousBusinessDay` — scan forward/backward for the nearest working day
+- `addBusinessDays` / `subtractBusinessDays` — advance or retreat by an exact count of working days
+- `businessDaysBetween` — count working days in a half-open interval `[start, end)`
+- Configurable `weekendDays` (any subset of 0–6)
+- Holiday exclusion via an `Date[]` array
+- Cooperative cancellation via `AbortSignal` (checked every 256 iterations in `businessDaysBetween`)
+- OTel counter via `recordBusinessDaysBetween`
+
+## Usage: Basic Example
+
+```ts
+import { isBusinessDay, addBusinessDays } from "@suaveplan/datetime/business-days";
+
+// Simple weekday check
+console.log(isBusinessDay("2024-03-18")); // true (Monday)
+console.log(isBusinessDay("2024-03-16")); // false (Saturday)
+
+// Advance 5 business days
+const result = addBusinessDays("2024-03-18", 5);
+console.log(result.toDateString()); // Fri Mar 29 2024 (skips weekend)
+```
+
+## Usage: Advanced Example
+
+```ts
+import {
+  businessDaysBetween,
+  nextBusinessDay,
+} from "@suaveplan/datetime/business-days";
+
+const nyseHolidays = [
+  new Date("2024-01-01"), // New Year's Day
+  new Date("2024-07-04"), // Independence Day
+];
+
+// How many NYSE trading days in Q1 2024?
+const q1 = businessDaysBetween("2024-01-01", "2024-04-01", {
+  holidays: nyseHolidays,
+});
+console.log(q1); // ~62 trading days
+
+// Cooperative cancellation
+const controller = new AbortController();
+setTimeout(() => controller.abort(), 10);
+const days = businessDaysBetween("2020-01-01", "2030-01-01", {
+  signal: controller.signal,
+});
+console.log(days); // partial count, aborted after ~256 iterations
+```
+
+## API Reference
+
+### `isBusinessDay(date: DateInput, opts?: BusinessDayOptions): boolean`
+
+Returns `true` when `date` is not a weekend day and not a holiday.
+
+- `opts.weekendDays` — default `[0, 6]` (Sun, Sat)
+- `opts.holidays` — list of `Date` objects to exclude
+
+### `nextBusinessDay(date: DateInput, opts?: BusinessDayOptions): Date`
+
+Returns the first business day strictly after `date`.
+
+### `previousBusinessDay(date: DateInput, opts?: BusinessDayOptions): Date`
+
+Returns the first business day strictly before `date`.
+
+### `addBusinessDays(date: DateInput, n: number, opts?: BusinessDayOptions): Date`
+
+Advance `date` by exactly `|n|` business days. Negative `n` moves backward.
+
+### `subtractBusinessDays(date: DateInput, n: number, opts?: BusinessDayOptions): Date`
+
+Move `date` backward by exactly `n` business days. Delegates to `addBusinessDays(date, -n)`.
+
+### `businessDaysBetween(start: DateInput, end: DateInput, opts?: BusinessDayOptions): number`
+
+Count business days in `[start, end)`. Returns a negative number when `end < start`. Checks `opts.signal` every 256 iterations.
+
+## Implementation Notes
+
+Holiday lookup uses a `Set<string>` keyed by `"YYYY-M-D"` (local time) built once per call. This is O(1) per day check and avoids allocating `Date` objects for the comparison.
+
+All date arithmetic is performed at local-midnight epoch millis to prevent time-of-day accumulation errors across DST transitions. The `midnight()` helper normalises any input to 00:00:00.000 local time before iteration begins.
+
+`addBusinessDays(d, 0)` returns the local midnight of `d` regardless of whether `d` itself is a business day, because zero steps means no movement.