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/web-ui/shape.md

@@ -3,7 +3,6 @@
 > Part of [Web UI](../web-ui.md). See also:
 > [Dashboard layout](dashboard.md) · [Per-agent page](agent.md)
 
-
 - `GET /` → `index.html` from the bundled frontend dist (see
   `frontend/`). Both binaries' routers declare their dynamic
   endpoints first and then `fallback_service(ServeDir::new(...))`
@@ -169,8 +168,8 @@ subscribes to `streamUrl` (live tail). Both endpoints return
 `.no-anim` class so they don't stagger in like live events. The
 optional `streamFactory(url)` callback lets the dashboard hand
 the factory a `SharedWorker`-backed `EventSource` facade (so
-multiple tabs share one upstream connection — see *SSE
-multiplexing* above); when omitted, the factory falls back to a
+multiple tabs share one upstream connection — see _SSE
+multiplexing_ above); when omitted, the factory falls back to a
 plain `new EventSource(url)`.
 
 **`linkify` (text-node based).** Bare `http(s)://` URLs in row
@@ -298,10 +297,10 @@ without its last path segment.
 That makes the page work under any prefix the agent ends up mounted
 at without rebuilding the dist. The cases that matter:
 
-| served at | `api/state` resolves to |
-|---|---|
-| `/` (own port, today's shape) | `/api/state` |
-| `/agent/iris/` (gateway-prefixed) | `/agent/iris/api/state` |
+| served at                                        | `api/state` resolves to |
+| ------------------------------------------------ | ----------------------- |
+| `/` (own port, today's shape)                    | `/api/state`            |
+| `/agent/iris/` (gateway-prefixed)                | `/agent/iris/api/state` |
 | `/agent/iris/stats` (subpage, no trailing slash) | `/agent/iris/api/state` |
 
 The gateway upstream config strips the prefix before forwarding to
@@ -316,4 +315,3 @@ the relative-path resolution stays correct: `static/app.js` from
 replaced), not `/stats/static/app.js`. Adding a trailing slash to
 those routes would break the resolution; either keep them
 slash-less or use `<base href>` injection at serve time.
-