fix(tests): exclude build/ from bats test discovery

Why
tests/run-bats.sh's discovery walk already excludes apm_modules/ and
.claude/skills/ because those hold apm-installed copies of the same
*.bats files one directory level shallower than their plugins/*/.apm/
source, which overshoots the hardcoded six-levels-up REPO_ROOT walk
each test's setup() does and fails to find the bats-support helper.
build/ was missing the same exclusion: apm pack stages an identical
copy under build/<package>-<version>/ before archiving, hitting the
exact same failure mode from a different apm subcommand. A stray
local `apm pack` run leaves that directory on disk (gitignored,
regenerable) and silently doubles the suite (846 tests instead of
423) with 423 of them failing.

Implementation Notes
Added `-not -path "*/build/*"` to the find walk and the matching
git ls-files grep exclusion, mirroring the existing apm_modules/ and
.claude/skills/ entries. Extended tests/test-run-bats.sh with a case
following the same pattern as the existing exclusion-bug fixtures.

Impact
Unblocks the run-tests pre-commit/pre-push hook for any checkout that
has ever run a bare `apm pack` locally.

tests/run-bats.sh

@@ -46,6 +46,17 @@ fi
 # which is ADR-0024 consequence 2 arriving here. Keeping the exclusion now is
 # what stops that landing as a mystery double-run on the merge that enables it.
 #
+# build/ is excluded for the same reason again, one layer further out: `apm
+# pack` stages a full copy of a package's tree (including its skills' tests/
+# directories) under build/<package>-<version>/ before archiving it. Those
+# staged .bats files carry the same six-levels-up REPO_ROOT walk-up as any
+# other copy, which resolves past this repo's actual root and fails on a
+# missing bats-support helper -- the same failure mode apm_modules/ and
+# .claude/skills/ above already guard against, just from a different apm
+# subcommand. build/ is gitignored and regenerated on demand, so nothing here
+# depends on its contents; the exclusion only stops a stray local `apm pack`
+# output from being discovered and double-run.
+#
 # The walk runs from inside REPO_ROOT so the exclusions match paths RELATIVE to
 # it, the same universe the `git ls-files` grep below sees. Matched against
 # absolute paths, `*/.claude/worktrees/*` excluded every file whenever the
@@ -61,6 +72,7 @@ done < <(
         -not -path "*/.claude/worktrees/*" \
         -not -path "*/apm_modules/*" \
         -not -path "*/.claude/skills/*" \
+        -not -path "*/build/*" \
         | sort
 )
 
@@ -99,7 +111,7 @@ if [[ -n "$GIT_TOPLEVEL" && "$GIT_TOPLEVEL" == "$REPO_ROOT" ]]; then
         [[ -n "$f" ]] && EXPECTED_FILES+=("$REPO_ROOT/$f")
     done < <(
         git -C "$REPO_ROOT" ls-files -- '*.bats' \
-            | grep -Ev '(^|/)tests/bats/|(^|/)test_helper/|(^|/)\.claude/worktrees/|(^|/)apm_modules/|(^|/)\.claude/skills/' \
+            | grep -Ev '(^|/)tests/bats/|(^|/)test_helper/|(^|/)\.claude/worktrees/|(^|/)apm_modules/|(^|/)\.claude/skills/|(^|/)build/' \
             | sort || true
     )
 else