docs: add CHANGELOG.md covering every release so far

Add a changelog from the first release (2026.10.12.1) up to
2026.10.15.3, newest first, with Added/Changed/Removed/Fixed sections
and the commit behind each entry. List dark mode under 2026.10.15.3,
since it shipped without a version bump.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01JZPd3GGWyxTgAkL6a4gAJz

CHANGELOG.md

@@ -0,0 +1,103 @@
+# Changelog
+
+Notable changes to kubestronaut-sim, newest first. A version is the `VERSION` string in `ksim`
+(year.month.day.n); `./ksim help` and the top bar of the web page show the one you are running.
+Every entry names the commit it came in.
+
+## 2026.10.15.3 — 2026-10-15
+
+### Added
+
+- **Browser UI in the style of [CK-X](https://github.com/sailor-sh/CK-X)** (`bab85d1`), itself
+  modelled on the Linux Foundation exam UI. Written from scratch, with nothing loaded from the
+  network.
+  - **Landing page.** The track switch sits in the top bar. **Start exam** opens a dialog to pick
+    the session: the next step on the path, an objective, a domain, the next domain, the whole bank,
+    tasks of your choice, a simulator set or a full exam, with or without hard mode. The full exam
+    warns while a domain has not been drilled yet. Below the dialog button come the path, a card per
+    domain, and every objective with its notes and skip links. While a session is prepared, an
+    overlay shows the clusters' progress and log; it comes back if the page is reloaded meanwhile.
+  - **Exam screen.** A dark top bar holds the clock and three menus: *Exam Interface* (fullscreen,
+    panel sizes, reconnect the desktop), *Exam Controls* (submit, pause, restart the clock,
+    terminate the session) and *Exam Information* (exam instructions, Kubernetes docs, history).
+    The light question panel has `< Question N of M >`, with flags and grades marked. Each task is
+    headed *Solve this question on instance: `ssh host`*, and ends with **Flag for review**,
+    **Satisfied with answer →** and, in drills, **Check this task**. The remote desktop sits beside
+    it, with reconnect and fullscreen buttons and a divider you can drag.
+  - **Results page.** Points, pass or fail, bars per domain, every task's checks and reference
+    solution, *View answers*, *Drill what I missed*, *Review tasks* and *History*.
+  - A session started in the browser opens a "Ready to begin" dialog, and its **Start** puts the
+    full time on the clock.
+- **Terminate session** and **Delete the hosts** (`bab85d1`), backed by `POST /api/terminate`. An
+  open session ends ungraded and nothing is recorded, as with `./ksim abort`; a finished session
+  only loses its hosts. Either way, only that session's hosts are deleted. The session payload now
+  says whether a session was aborted.
+- **Dark mode** (`db2575b`): ◐ Auto (your system's setting, the default), ☀ Light or ☾ Dark. The
+  switch is in the landing page's top bar, the *Exam Interface* menu and on the results page. The
+  browser keeps the choice and applies it before the page draws. This came without a version bump.
+
+### Changed
+
+- The landing page, exam screen and results page replace the lobby overlay, the number rail with
+  its task pane, and the old results overlay. The browser remembers hard mode and the width of the
+  question panel.
+
+## 2026.10.15.2 — 2026-10-15
+
+### Added
+
+- **`./ksim restart` and a Restart clock button** (`c244bd9`). The session's full time goes back on
+  the clock and starts running; the hosts, the work on them, results, flags and notes stay. Like
+  pause, it is recorded nowhere: the attempt's time used counts from the restart. A submitted
+  session refuses it. The button is backed by `POST /api/restart`.
+
+## 2026.10.15.1 — 2026-10-15
+
+### Removed
+
+- **Study mode** (`c266eeb`): there are no more untimed sessions with the reference solutions open
+  from the start, and `./ksim study` is gone. Every session runs against the clock.
+
+### Changed
+
+- The path is objective drills against the clock, then a mock, then the fixed simulator sets. An
+  objective is done when every task passed on its latest timed attempt; the "studied" stage and its
+  `~` mark are gone. `./ksim next` always drills, and the lobby's "Study mode" became "Drill an
+  objective".
+- `study-<track>.json` keeps your focus, the domains you marked known and when each objective was
+  done. Results of old study sessions stay in it, read by nothing.
+
+### Added
+
+Work from 2026-10-13, first committed here:
+
+- **`./ksim skip` / `unskip`**, and the lobby's "I know this — skip it" links. They mark a domain or
+  an objective as known, so the path, the drill with no argument and `drill all` leave it out. A
+  mock still draws from every domain.
+- **`DOMAIN:N`** picks N tasks of a domain spread over its objectives: `./ksim drill OBS:3`, or
+  `./ksim next OBS:3` to add them to the next step.
+- A command with neither `--track` nor `KSIM_TRACK` is for the track whose session is under way.
+  `./ksim show` says where a task's host resolves.
+
+### Fixed
+
+- CKAD Q60: its check now says which file holds which run's output.
+
+## 2026.10.12.1 — 2026-10-13
+
+The first release (`f3d0e53`), grown out of `cka-sim`.
+
+- **One host per task.** Each task gets a kind (kubeadm) cluster in rootless podman, with real etcd,
+  a static-pod control plane and a systemd kubelet. You reach it with `ssh <host>` from a remote
+  desktop in the browser (XFCE, a terminal, Firefox limited to the documentation the exam allows).
+- **CKS hosts** carry kube-bench, trivy, kubesec, kube-linter, bom, cosign, istioctl, gVisor and
+  Kubernetes from apt packages. The Falco and AppArmor tasks run on QEMU/KVM Ubuntu VMs.
+- **A pull-through image cache** (`ksim-registry`) keeps sessions under Docker Hub's pull limit.
+- **Task banks:** CKA 131, CKAD 105 and CKS 110 tasks, with at least 17 pool tasks in every domain.
+  Each track comes with its curriculum, study notes, reference solutions and scripted solutions.
+- Drills, full mocks with hard mode, the fixed simulator sets (CKA C and D, CKAD A), a curriculum
+  path with study mode, attempt history and weak spots.
+- `./ksim coverage` and `./ksim audit`. All 346 tasks pass the audit: each seeds cleanly, scores
+  nothing untouched and reaches full marks with its scripted solution.
+
+The repository's initial commit (`3d7605c`) holds the licence and a stub README.