fix(ci): build Hypatia escript from repo root (estate dogfood drift)

The hypatia repo dropped its scanner/ subdirectory; mix.exs now lives at
the repo root. The dogfood build step still did `cd scanner`, failing with
"cd: scanner: No such file or directory" and red-flagging every PR.
Build from the checkout root instead. Idempotent estate sweep.

Refs verisimiser#59

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: hyperpolymath

.github/workflows/hypatia-scan.yml

@@ -45,7 +45,6 @@ jobs:
         run: |
           if [ ! -f hypatia-v2 ]; then
             echo "Building hypatia-v2 scanner..."
-            cd scanner
             mix deps.get
             mix escript.build
             mv hypatia ../hypatia-v2
@@ -231,4 +230,4 @@ jobs:
               repo: context.repo.repo,
               issue_number: context.issue.number,
               body: comment
-            });
+            });