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/blocked.txt

@@ -4,7 +4,7 @@ bd init --prefix test
 # Create first issue
 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'
 
 # Create second issue that depends on first
 exec sh -c 'bd create "Second issue" --deps $(cat first_id.txt)'

cmd/bd/testdata/close.txt

@@ -4,7 +4,7 @@ bd init --prefix test
 # 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'
+exec sh -c 'grep -oE "test-[a-z0-9]+" issue.txt > issue_id.txt'
 
 # Close the issue
 exec sh -c 'bd close $(cat issue_id.txt) --reason Fixed'

cmd/bd/testdata/dep_add.txt

@@ -11,8 +11,8 @@ cp stdout second.txt
 grep 'Created issue: test-' second.txt
 
 # Extract IDs using grep (hash IDs are test-XXXXXXXX format)
-exec sh -c 'grep -oE "test-[a-f0-9]+" first.txt > first_id.txt'
-exec sh -c 'grep -oE "test-[a-f0-9]+" second.txt > second_id.txt'
+exec sh -c 'grep -oE "test-[a-z0-9]+" first.txt > first_id.txt'
+exec sh -c 'grep -oE "test-[a-z0-9]+" second.txt > second_id.txt'
 
 # Add dependency: second depends on first
 exec sh -c 'bd dep add $(cat second_id.txt) $(cat first_id.txt)'

cmd/bd/testdata/dep_remove.txt

@@ -4,11 +4,11 @@ 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'
+exec sh -c 'grep -oE "test-[a-z0-9]+" first.txt > first_id.txt'
 
 bd create 'Second issue'
 cp stdout second.txt
-exec sh -c 'grep -oE "test-[a-f0-9]+" second.txt > second_id.txt'
+exec sh -c 'grep -oE "test-[a-z0-9]+" second.txt > second_id.txt'
 
 # Add dependency
 exec sh -c 'bd dep add $(cat second_id.txt) $(cat first_id.txt)'

cmd/bd/testdata/dep_tree.txt

@@ -4,11 +4,11 @@ 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'
+exec sh -c 'grep -oE "test-[a-z0-9]+" root.txt > root_id.txt'
 
 bd create 'Child issue'
 cp stdout child.txt
-exec sh -c 'grep -oE "test-[a-f0-9]+" child.txt > child_id.txt'
+exec sh -c 'grep -oE "test-[a-z0-9]+" 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)'

cmd/bd/testdata/show.txt

@@ -7,7 +7,7 @@ cp stdout issue.txt
 grep 'Created issue: test-' issue.txt
 
 # Extract ID using grep
-exec sh -c 'grep -oE "test-[a-f0-9]+" issue.txt > issue_id.txt'
+exec sh -c 'grep -oE "test-[a-z0-9]+" issue.txt > issue_id.txt'
 
 # Show the issue
 exec sh -c 'bd show $(cat issue_id.txt)'

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'
 

cmd/bd/testdata/update.txt

@@ -7,7 +7,7 @@ cp stdout issue.txt
 grep 'Created issue: test-' issue.txt
 
 # Extract ID using grep
-exec sh -c 'grep -oE "test-[a-f0-9]+" issue.txt > issue_id.txt'
+exec sh -c 'grep -oE "test-[a-z0-9]+" issue.txt > issue_id.txt'
 
 # Update the issue status
 exec sh -c 'bd update $(cat issue_id.txt) --status in_progress'