Resolve the gate's cache dir with --git-common-dir, and say when it cannot (#258)

CACHE_DIR was the literal ".git/lmc-verify". In a linked worktree `.git` is a
FILE containing `gitdir: ...`, so `mkdir -p .git/lmc-verify` fails with "Not a
directory" -- and because the write is the last thing the script does, it failed
while the gate still printed green and exited 0. Every commit and push from a
worktree then re-swept API 33-36 for nothing, silently. That is the worst shape a
cache can fail in: invisible and expensive, and it was found by an agent paying
for it four times over rather than by the tool saying anything.

Measured both ways: in a worktree the old expression gives
`mkdir: cannot create directory '.git': Not a directory`, exit 1; `git rev-parse
--git-common-dir` gives the real path and exit 0.

--git-common-dir rather than --git-dir so the cache is SHARED between worktrees.
The key is the app/src tree hash, and identical content is identical content
whichever worktree produced it -- a sweep run in one is evidence for all of them.

The write also stops being silent. record_sweep() prints when it cannot record,
because a cache that never fills looks exactly like one that is working.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

tools/git-hooks/local-gate.sh

@@ -44,10 +44,28 @@ cd "$REPO_ROOT" || exit 1
 
 MODE="$(basename "$0")"
 ZERO="0000000000000000000000000000000000000000"
-CACHE_DIR=".git/lmc-verify"
+# `git rev-parse --git-common-dir`, not a literal ".git" (#258). In a linked worktree `.git` is a
+# FILE containing `gitdir: ...`, so `mkdir -p .git/lmc-verify` fails with "Not a directory" -- and
+# because the write is the last thing this script does, it failed while the gate still printed
+# green and exited 0. Every push from a worktree then re-swept 33-36 for nothing, silently, which
+# is the worst shape a cache can fail in: invisible and expensive.
+#
+# --git-common-dir rather than --git-dir so the cache is SHARED across worktrees. The key is the
+# app/src tree hash, and identical content is identical content whichever worktree produced it.
+CACHE_DIR="$(git rev-parse --git-common-dir)/lmc-verify"
 GRADLE_GATE=(:app:assembleDebug :app:testDebugUnitTest :app:compileDebugAndroidTestKotlin
   :app:ktlintCheck :app:detekt :app:lintDebug)
 
+# Says so when it cannot record, rather than leaving a cache that silently never fills (#258).
+record_sweep() {
+  [ -n "$tree" ] || return 0
+  if mkdir -p "$CACHE_DIR" 2>/dev/null && : > "$CACHE_DIR/$tree" 2>/dev/null; then
+    return 0
+  fi
+  printf '\n\033[1m[local-gate]\033[0m could not record the sweep under %s -- it will re-run next
+  time. Not fatal, but it means every commit and push pays for it again.\n' "$CACHE_DIR"
+}
+
 say() { printf '\n\033[1m[local-gate]\033[0m %s\n' "$*"; }
 die() {
   printf '\n\033[1;31m[local-gate] BLOCKED\033[0m %s\n' "$*"
@@ -180,7 +198,7 @@ fi
 
 if [ "$touches_source" -eq 0 ] && [ "$touches_tests" -eq 0 ]; then
   say "no app/src changes; the instrumented sweep is not required for this one"
-  mkdir -p "$CACHE_DIR" && [ -n "$tree" ] && : > "$CACHE_DIR/$tree"
+  record_sweep
   exit 0
 fi
 
@@ -223,7 +241,7 @@ else
   attach the Pixel 10 Pro XL to have this hook cover it too."
 fi
 
-mkdir -p "$CACHE_DIR" && [ -n "$tree" ] && : > "$CACHE_DIR/$tree"
+record_sweep
 # Name the levels rather than claiming "every supported level". The first cut said the latter on
 # both paths, including the one that had just printed NOT COVERED two lines above -- a false claim
 # printed by the tool whose whole job is to stop false claims reaching CI.