feat(TASK-048): install polish, hooks-dir mixin scan, script and probe hardening

- one shared exec-bit predicate (Language-recognized extensions => 0755) used by
  every hook/agent installer; non-script files in hooks dirs stay non-executable
- mixin discovery scans the global hooks dir and each agent's hooks dir;
  .builtin-manifest is never scanned
- NTFS 8.3 short-name aliases rejected as builtin-manifest entries
- hook-script install announcements restricted to real hook locations
- COYOTE_HOOKS_DIR override warning compares canonicalized paths
- embedded assets decode via checked from_utf8 instead of from_utf8_unchecked
- log-events.sh defaults under ${XDG_STATE_HOME:-$HOME/.local/state}/coyote/hooks.log
  keeping 0600/symlink-refusal/secret-filter hardening; notify.sh tty fallback strips
  control bytes; both use UTC ISO-8601 timestamps
- payload files orphaned by a write failure are unlinked; stale coyote-hook-*.json
  payloads swept best-effort at startup
- sweep_removed_agent_hooks never follows symlinked agent dirs
- usage-probe-hooks.sh replaces bare sleeps with a bounded poll, re-verifies 0755 on
  restored scripts, and covers roles/agent hook locations
- TestConfigDirGuard hoisted into crate::testing
- .gitattributes pins LF line endings for bundled scripts

hooks is now a reserved bundle name.

assets/hooks/log-events.sh

@@ -4,7 +4,8 @@
 # each one carries.
 #
 # Env vars read:
-#   COYOTE_HOOK_LOG   log file to append to (default: /tmp/coyote-hooks.log)
+#   COYOTE_HOOK_LOG   log file to append to
+#                     (default: ${XDG_STATE_HOME:-$HOME/.local/state}/coyote/hooks.log)
 #   COYOTE_*          everything coyote sets for the event is dumped verbatim
 #
 # Secrets (COYOTE_SECRET_*) are excluded from the snapshot; widening the grep
@@ -26,15 +27,15 @@
 
 set -u
 
-log_file="${COYOTE_HOOK_LOG:-/tmp/coyote-hooks.log}"
+log_file="${COYOTE_HOOK_LOG:-${XDG_STATE_HOME:-$HOME/.local/state}/coyote/hooks.log}"
 
-# The default lives in shared /tmp: refuse to append through a symlink someone
-# else planted there. Point COYOTE_HOOK_LOG at a private path to avoid the
-# shared directory entirely.
+# COYOTE_HOOK_LOG may point into a shared directory: refuse to append through
+# a symlink someone else planted there.
 [ -L "$log_file" ] && exit 0
 umask 077
+mkdir -p "$(dirname "$log_file")" 2> /dev/null || true
 {
-  echo "=== $(date '+%Y-%m-%dT%H:%M:%S%z') ${COYOTE_EVENT:-unknown}"
+  echo "=== $(date -u '+%Y-%m-%dT%H:%M:%SZ') ${COYOTE_EVENT:-unknown}"
   env | grep '^COYOTE_' | grep -v '^COYOTE_SECRET_' | sort
   echo
 } >> "$log_file" 2> /dev/null || true