Switch from hex to Base36 encoding for issue IDs (GH #213)

This change improves information density by using Base36 (0-9, a-z) instead
of hex (0-9, a-f) for hash-based issue IDs. Key benefits:

- Shorter IDs: Can now use 3-char IDs (was 4-char minimum)
- Better scaling: 3 chars good for ~160 issues, 4 chars for ~980 issues
- Case-insensitive: Maintains excellent CLI usability
- Backward compatible: Old hex IDs continue to work

Changes:
- Implemented Base36 encoding with proper truncation (keep LSB)
- Updated adaptive length thresholds (3-8 chars instead of 4-8)
- Fixed collision probability math to match encoding (was calculating
  for base36 but encoding in hex - now both use base36)
- Fixed ID parser bug (use prefixWithHyphen for substring matching)
- Updated all tests and test data patterns

Fixes #213

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

cmd/bd/testdata/stats.txt

@@ -4,7 +4,7 @@ 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'
+exec sh -c 'grep -oE "test-[a-z0-9]+" first.txt > first_id.txt'
 
 bd create 'Second issue'