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/close.txt

@@ -1,8 +1,15 @@
 # Test bd close command
 bd init --prefix test
-bd create 'Issue to close'
-bd close test-1 --reason 'Fixed'
-stdout 'Closed test-1'
 
-bd show test-1
+# Create issue and capture its hash ID
+bd create 'Issue to close'
+cp stdout issue.txt
+exec sh -c 'grep -oE "test-[a-f0-9]+" issue.txt > issue_id.txt'
+
+# Close the issue
+exec sh -c 'bd close $(cat issue_id.txt) --reason Fixed'
+stdout 'Closed test-'
+
+# Verify it's closed
+exec sh -c 'bd show $(cat issue_id.txt)'
 stdout 'closed'