Add a plain-language changelog from inception (#91)

Human-readable history of the bot from launch (Apr 3) through v1.9.2, written
for a non-technical reader. Recent entries map to the new semantic version tags
(v1.7.19-v1.9.2); earlier work is grouped by dated milestone since those
releases were daily auto-versioned.

CHANGELOG.md

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+# Changelog
+
+What's changed in the **After-Hours Shift Manager** — the Slack bot that runs our
+after-hours on-call phone duty. Newest first, in plain language.
+
+Versions are `MAJOR.MINOR.PATCH`: a **minor** bump adds a new feature, a **patch**
+is a fix or tidy-up, and a **major** would be a big change to how things work.
+
+---
+
+## v1.9.2 — June 1, 2026
+
+**Setup-guide fix.** Corrected the install instructions so secrets (the Slack and
+phone-system passwords) are stored in the right, secure place. No change to the
+bot itself — this only affects someone setting it up from scratch.
+
+## v1.9.0 / v1.9.1 — June 1, 2026
+
+**You can no longer drop a shift at the last minute.** If a shift starts within
+the next 24 hours, you can't just drop it and walk away — you have to hand it to
+someone specific (a swap they accept), or ask an admin. This stops the phones
+from being left uncovered with no notice. *(v1.9.1 was a routine update of
+behind-the-scenes software libraries.)*
+
+## v1.8.0 — June 1, 2026
+
+**Swaps now need the other person to say yes.** Before, `/oncall swap` instantly
+dumped your shift on someone else. Now they get a Slack message with **Accept**
+and **Decline** buttons, and the shift only moves if they accept. Until then it
+stays yours. If they don't respond before the shift starts, the request quietly
+expires.
+
+## v1.7.19 — June 1, 2026
+
+**Quality safety net (no visible change).** Added an automated test suite that
+checks the bot's behavior on every change, so bugs get caught before they ship.
+You won't notice anything different day-to-day — it just makes future updates
+safer.
+
+---
+
+## May 2026 — Phone-system automation & a big fix
+
+- **Live phone routing follows the schedule.** The system that decides which
+  phone rings after hours now reads directly from the on-call schedule, so
+  pickups, drops, and swaps take effect automatically.
+- **Fixed a release that had broken the whole bot.** A packaging mistake stopped
+  all of the bot's functions from running; this was found and fixed.
+- Ongoing routine updates to behind-the-scenes software libraries.
+- **Behind the scenes:** moved to an automated build-and-release pipeline, added
+  automatic code checks and formatting, and turned on automated dependency and
+  code-review tooling. None of this changes how you use the bot.
+
+## May 1, 2026 — Reliability & notifications
+
+- **No more double-booking.** Fixed a timing bug where two people could grab the
+  same open shift at once.
+- **Shift changes are announced.** When someone picks up, drops, or swaps a
+  shift, a note is posted to the team channel.
+- **Fixed a confusing error** that showed up when picking a shift even though the
+  pickup had actually worked.
+
+## April 8, 2026 — Schedule & pay polish
+
+- The weekly schedule now always starts on **Monday** (it used to start from
+  today).
+- Tidied up the weekly pay report (recipient and wording).
+
+## April 7, 2026 — Scheduling & pay
+
+- **Two-week schedule view** instead of just the current week.
+- **Weekly pay totals** for on-call shifts, with a configurable flat rate.
+- **Per-person pay rates** and an `/oncall rate` command to manage them from
+  Slack.
+- **Weekend day shifts** — weekends are split into a daytime and an overnight
+  shift.
+- **Automatic employee roster sync** — the bot pulls the staff list from the
+  phone system each day, so the roster stays current on its own.
+- **Weekly "Bonus Pay Summary"** emailed to payroll (and sent as a Slack DM to
+  the admin) every Monday.
+
+## April 3, 2026 — Launch 🎉
+
+The first version of the After-Hours Shift Manager:
+
+- See the on-call schedule in Slack with `/oncall`.
+- **Pick up** an open shift and **drop** a shift you're covering.
+- Link your Slack account to your phone extension with `/oncall register`.