Remove obsolete N-way collision tests, add focused hash ID tests

- Remove beads_nway_test.go (92s of obsolete sequential ID collision tests)
- Remove beads_twoclone_test.go (already skipped tests)
- Add beads_hash_multiclone_test.go (6s, tests hash ID multi-clone sync)
- Fix init.go git hook to remove --resolve-collisions flag

Result: 87% faster test suite (96s → 12s for full suite)

Hash-based IDs prevent collisions by design, making extensive N-way
collision resolution tests unnecessary. New tests verify that:
- Multiple clones can sync without ID collisions
- Identical content deduplicates correctly

Amp-Thread-ID: https://ampcode.com/threads/T-b256a7ad-c279-4c87-8b6b-6c34c6f05e7f
Co-authored-by: Amp <amp@ampcode.com>

cmd/bd/init.go

@@ -404,9 +404,9 @@ fi
 # Import the updated JSONL
 # The auto-import feature should handle this, but we force it here
 # to ensure immediate sync after merge
-if ! bd import -i .beads/issues.jsonl --resolve-collisions >/dev/null 2>&1; then
+if ! bd import -i .beads/issues.jsonl >/dev/null 2>&1; then
     echo "Warning: Failed to import bd changes after merge" >&2
-    echo "Run 'bd import -i .beads/issues.jsonl --resolve-collisions' manually" >&2
+    echo "Run 'bd import -i .beads/issues.jsonl' manually" >&2
     # Don't fail the merge, just warn
 fi