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/ci.md

@@ -7,6 +7,7 @@ The `hive-ci` module runs a Forgejo Actions runner in a `hive-ci` nixos-containe
 Set `services.hyperhive.forge.ci.enable = true` in the host NixOS config. That's it — no manual token provisioning.
 
 **Requirements:**
+
 - The internal forge is always present (mandatory), so the runner always has a hive-forge instance to register against — nothing extra to enable.
 - Optional: tune `services.hyperhive.forge.ci.name` (runner name in forge admin panel), `concurrency` (parallel job capacity), `labels` (workflow targeting), `jobTimeout` (per-job wall-clock cap, default `"1h"`, Go duration string e.g. `"3h"` — a job that exceeds it is killed so a hung or runaway build can't hold the runner's single slot indefinitely).
 
@@ -148,7 +149,7 @@ following to your host config:
 ```
 
 **Remote builders:** if CI dispatches builds to a remote builder (e.g. via
-`nix.buildMachines` / `ssh-ng://`), the build outputs land in *that host's*
+`nix.buildMachines` / `ssh-ng://`), the build outputs land in _that host's_
 store, so the same GC config should be applied wherever the builder runs —
 GC on the coordinator host won't reclaim space on the builder.