feat(#1997): add prettier markdown formatter to treefmt

- .prettierrc: proseWrap=preserve (no prose reflow)
- .prettierignore: exclude hivectl-cli.md (auto-generated) + 11 docs
  with multi-line list-item continuations prettier would strip to col 0
  (CommonMark limitation in prettier's list handling)
- format 16 markdown files: cosmetic only (*→_, table alignment,
  heading normalisation) — verified no broken continuations, idempotent

docs/gotchas.md

@@ -201,7 +201,7 @@ Never use raw `curl` for forge access.
 
 Agent containers bind-mount the host's nix-daemon socket. nspawn
 containers don't get user-namespaces by default, so `nix build`
-invocations *inside* the container can't set up the build sandbox
+invocations _inside_ the container can't set up the build sandbox
 and fail outright if the host daemon's
 `nix.settings.sandbox-fallback` is `false` (nixpkgs default).
 `nix/templates/harness-base.nix` does `lib.mkForce true` so builds
@@ -277,7 +277,7 @@ connects to the compositor at `127.0.0.1:<vnc_port>`.
   ("Failed to connect to the bus" → binds `xdg_wm_base` then destroys it
   = invisible window even though CDP works). The fix is **not** to wrap
   each client in its own `dbus-run-session` (a private throwaway bus per
-  process — that's a *separate* session, defeating the one-session
+  process — that's a _separate_ session, defeating the one-session
   model); it's this one shared bus, whose address is exported as
   `DBUS_SESSION_BUS_ADDRESS=unix:path=/run/gui/bus` via
   `systemd.globalEnvironment` so weston, the harness and every GUI client