Remove obsolete collision remapping code and tests

- Deleted collision remapping tests (obsolete with hash IDs bd-8e05)
- Simplified collision.go from 704 to 138 lines
- Removed RemapCollisions, ScoreCollisions, and reference update code
- Removed issue_counters table dependencies (bd-807b)
- Added COLLISION_MATH.md documentation
- Fixed RenameCounterPrefix and ResetCounter to be no-ops
- Closed bd-a58f, bd-3d65, bd-807b

Hash-based IDs make collision remapping unnecessary since collisions
are extremely rare (same ID = same content).

Amp-Thread-ID: https://ampcode.com/threads/T-cbb0f111-6a95-4598-b03e-c137112f9875
Co-authored-by: Amp <amp@ampcode.com>

cmd/bd/testdata/stats.txt

@@ -1,8 +1,17 @@
 # Test bd stats command
 bd init --prefix test
+
+# Create issues
 bd create 'First issue'
+cp stdout first.txt
+exec sh -c 'grep -oE "test-[a-f0-9]+" first.txt > first_id.txt'
+
 bd create 'Second issue'
-bd close test-1
+
+# Close one issue
+exec sh -c 'bd close $(cat first_id.txt)'
+
+# Check stats
 bd stats
 stdout 'Total Issues:'
 stdout 'Open:'