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_tree.txt

@@ -1,7 +1,18 @@
 # Test bd dep tree command
 bd init --prefix test
+
+# Create issues and capture their hash IDs
 bd create 'Root issue'
+cp stdout root.txt
+exec sh -c 'grep -oE "test-[a-f0-9]+" root.txt > root_id.txt'
+
 bd create 'Child issue'
-bd dep add test-2 test-1
-bd dep tree test-1
-stdout 'test-1'
+cp stdout child.txt
+exec sh -c 'grep -oE "test-[a-f0-9]+" child.txt > child_id.txt'
+
+# Add dependency: child depends on root
+exec sh -c 'bd dep add $(cat child_id.txt) $(cat root_id.txt)'
+
+# Show dependency tree for root
+exec sh -c 'bd dep tree $(cat root_id.txt)'
+stdout 'test-'