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/dep_remove.txt

@@ -1,10 +1,22 @@
 # Test bd dep remove command
 bd init --prefix test
+
+# Create issues and capture their hash IDs
 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 dep add test-2 test-1
-bd dep remove test-2 test-1
+cp stdout second.txt
+exec sh -c 'grep -oE "test-[a-f0-9]+" second.txt > second_id.txt'
+
+# Add dependency
+exec sh -c 'bd dep add $(cat second_id.txt) $(cat first_id.txt)'
+stdout 'Added dependency'
+
+# Remove dependency
+exec sh -c 'bd dep remove $(cat second_id.txt) $(cat first_id.txt)'
 stdout 'Removed dependency'
 
-bd show test-2
-! stdout 'test-1'
+# Verify dependency is gone
+exec sh -c 'bd show $(cat second_id.txt) | grep -v "Depends on"'