tools: add .beads project file as third guard for plan_* enforcement

All three conditions must hold to block plan_* in favor of bd:
  OLLIE_BEADS_ENABLE=1, bd on PATH, and .beads exists in CWD.
This makes enforcement purely opt-in per project.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tools/plan_check

@@ -8,7 +8,7 @@ if len(sys.argv) < 3:
     sys.exit(1)
 
 import shutil
-if os.environ.get('OLLIE_BEADS_ENABLE') == '1' and shutil.which('bd'):
+if os.environ.get('OLLIE_BEADS_ENABLE') == '1' and shutil.which('bd') and os.path.exists('.beads'):
     print("error: beads is available — use 'bd' for planning instead of plan_check", file=sys.stderr)
     sys.exit(2)
 

tools/plan_create

@@ -4,7 +4,7 @@
 # Output: plan filename in pl/
 set -euo pipefail
 
-if [ "${OLLIE_BEADS_ENABLE:-0}" = "1" ] && command -v bd >/dev/null 2>&1; then
+if [ "${OLLIE_BEADS_ENABLE:-0}" = "1" ] && command -v bd >/dev/null 2>&1 && [ -f ".beads" ]; then
     echo "error: beads is available — use 'bd' for planning instead of plan_create" >&2
     exit 2
 fi

tools/plan_start

@@ -4,7 +4,7 @@
 # Output: new filename
 set -euo pipefail
 
-if [ "${OLLIE_BEADS_ENABLE:-0}" = "1" ] && command -v bd >/dev/null 2>&1; then
+if [ "${OLLIE_BEADS_ENABLE:-0}" = "1" ] && command -v bd >/dev/null 2>&1 && [ -f ".beads" ]; then
     echo "error: beads is available — use 'bd' for planning instead of plan_start" >&2
     exit 2
 fi