feat(T3.5-T3.8): complete feature expansion tasks

T3.5: Custom Judge Implementations
- Add internal/judge package for custom judges
- Implement Judge interface for domain-specific validators
- CustomJudgeRegistry for managing judges
- Register/unregister judges at runtime
- Set default judge
- List all registered judges
- 5 judge tests, all passing

T3.6: Immutable Audit Trail (Enhanced)
- Add internal/audit/immutable_log.go for tamper-proof logging
- SHA256-based hash chaining for integrity
- Immutable append-only entry structure
- Entry sequencing and previous hash tracking
- Verify() for integrity checks
- Metadata storage for extensibility
- 4 immutable log tests, all passing

T3.7: Workflow Composition
- Add internal/composition package for nested workflows
- WorkflowComposer for managing child orchestrators
- ChildOrchestrator representing nested workflows
- Parent-child task relationships
- Status tracking for child workflows
- Hierarchy queries
- 4 composition tests, all passing

T3.8: External Task System Integration
- Add internal/external package for task importing
- TaskImporter for GitHub/Linear/JIRA task import
- Source tracking (github, linear, jira)
- Task status synchronization
- Query by source
- External ID mapping
- 5 external task tests, all passing

T3 Milestone: 8/8 tasks COMPLETE (100%)

Test Coverage:
- T3.5: 5 judge tests
- T3.6: 4 immutable log tests
- T3.7: 4 composition tests
- T3.8: 5 external task tests
- Total T3: 40+ tests across 8 tasks, all passing
- Combined with T1+T2: 240+ tests, zero failures

Architecture:
- Each T3 task is independent package with zero cross-dependencies
- Interfaces enable extension and testing
- Thread-safe concurrent operations
- Minimal external dependencies
- Production-ready implementations

Next: Prepare T1+T2+T3 for squash-merge to main

internal/audit/immutable_log_test.go

@@ -0,0 +1,47 @@
+package audit
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestAppendEntry(t *testing.T) {
+	log := NewImmutableLog("", "workflow-1")
+	entry, err := log.Append("Decision: approved", map[string]interface{}{})
+	
+	assert.NoError(t, err)
+	assert.NotNil(t, entry)
+	assert.Equal(t, int64(1), entry.Sequence)
+}
+
+func TestVerifyIntegrity(t *testing.T) {
+	log := NewImmutableLog("", "workflow-1")
+	
+	log.Append("Entry 1", map[string]interface{}{})
+	log.Append("Entry 2", map[string]interface{}{})
+	
+	valid, err := log.Verify()
+	assert.NoError(t, err)
+	assert.True(t, valid)
+}
+
+func TestGetEntries(t *testing.T) {
+	log := NewImmutableLog("", "workflow-1")
+	
+	log.Append("Entry 1", map[string]interface{}{})
+	log.Append("Entry 2", map[string]interface{}{})
+	
+	entries := log.GetEntries()
+	assert.Equal(t, 2, len(entries))
+}
+
+func TestChainHashes(t *testing.T) {
+	log := NewImmutableLog("", "workflow-1")
+	
+	entry1, _ := log.Append("Entry 1", map[string]interface{}{})
+	entry2, _ := log.Append("Entry 2", map[string]interface{}{})
+	
+	assert.Equal(t, "genesis", entry1.PrevHash)
+	assert.Equal(t, entry1.Hash, entry2.PrevHash)
+}