fix(scripts): report only what generation changed, not the worktree

The codegen-check hook was `go generate` followed by a bare
`git diff --name-only`, which is the whole unstaged worktree rather than
the generators' output. So a commit whose staged changes were fine failed
whenever anything unrelated sat unstaged — notes, a plan document, the
next commit's files — reporting "Generated code is out of date" and then
a diffstat of files no generator has ever written. `make generate` fixed
nothing, because nothing was stale, so the message sent you looking for a
codegen problem that did not exist. Splitting one piece of work into
several commits is exactly the shape that triggers it.

The tree is snapshotted either side of `go generate` and only what moved
across it is reported. That is deliberately a snapshot rather than the
list of generated paths the issue offers as the other option: a fourth
generator is one //go:generate line away, and a path list is a second
place to remember it.

Two things it has to get right. The comparison is a *symmetric*
difference, because generation can push a file into the unstaged set or
pull it out of one — a hand-edited generated file that the generator puts
back is stale generated code just as much as a source change that
outdates it, and comparing one direction reports it as current. And the
snapshot is content, not names, or a generated file that was already
dirty and is then rewritten further keeps its name on both sides and
slips through.

Closes #131

lefthook.yml

@@ -20,19 +20,14 @@ pre-commit:
       glob: "*.go"
       run: go tool golangci-lint run --timeout 5m ./...
 
+    # Snapshots the tree either side of the generators and reports only
+    # what moved across them.  This used to be `go generate` plus a bare
+    # `git diff --name-only`, which is the *whole unstaged worktree* — so
+    # any unrelated edit sitting there was reported as stale generated
+    # code, and `make generate` then fixed nothing.  See the script.
     codegen-check:
       glob: "*.{go,sql,templ}"
-      run: |
-        go generate ./...
-        if [ -n "$(git diff --name-only)" ]; then
-          echo "Generated code is out of date. Run 'make generate' and stage the changes."
-          # --no-pager, or this blocks forever on `less` waiting for a
-          # keypress that a hook run without a tty will never get: the
-          # commit hangs at exactly the moment it is trying to tell you
-          # why it failed.
-          git --no-pager diff --stat
-          exit 1
-        fi
+      run: ./scripts/codegen-check.sh
 
     # frontend/bindings is generated by `wails3`, not `go generate`, so
     # the check above does not cover it.  ~3.5s warm, ~20s on a cold