Add greptile.json: review non-draft PRs once, not per commit (#852)

Configure Greptile to review each PR once (on open / ready-for-review)
instead of on every commit, and to skip drafts.

- `triggerOnUpdates: false` — no re-review on new commits (Greptile
defaults this on); the initial review still happens.
- `triggerOnDrafts: false` — do not review draft PRs (Greptile default;
set explicitly).
- Keeps review noise to one pass per PR, matching how we want the repo's
review bots to behave.

---------

Signed-off-by: Clemens Volk <cvolk@nvidia.com>

Author: cvolkcvolk

greptile.json

@@ -0,0 +1,4 @@
+{
+  "triggerOnDrafts": false,
+  "triggerOnUpdates": false
+}