feat(storage): add VersionedStorage interface with history/diff/branch operations

Extends Storage interface with Dolt-specific version control capabilities:

- New VersionedStorage interface in storage/versioned.go with:
  - History queries: History(), AsOf(), Diff()
  - Branch operations: Branch(), Merge(), CurrentBranch(), ListBranches()
  - Commit operations: Commit(), GetCurrentCommit()
  - Conflict resolution: GetConflicts(), ResolveConflicts()
  - Helper types: HistoryEntry, DiffEntry, Conflict

- DoltStore implements VersionedStorage interface

- New CLI commands:
  - bd history <id> - Show issue version history
  - bd diff <from> <to> - Show changes between commits/branches
  - bd branch [name] - List or create branches
  - bd vc merge <branch> - Merge branch to current
  - bd vc commit -m <msg> - Create a commit
  - bd vc status - Show current branch/commit

- Added --as-of flag to bd show for time-travel queries

- IsVersioned() helper for graceful SQLite backend detection

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/storage/dolt/versioned_test.go

@@ -0,0 +1,24 @@
+package dolt
+
+import (
+	"testing"
+
+	"github.com/steveyegge/beads/internal/storage"
+)
+
+// TestDoltStoreImplementsVersionedStorage verifies DoltStore implements VersionedStorage.
+// This is a compile-time check.
+func TestDoltStoreImplementsVersionedStorage(t *testing.T) {
+	// The var _ declaration in versioned.go already ensures this at compile time.
+	// This test just documents the expectation.
+
+	var _ storage.VersionedStorage = (*DoltStore)(nil)
+}
+
+// TestVersionedStorageMethodsExist ensures all required methods are defined.
+// This is mostly a documentation test since Go's type system enforces this.
+func TestVersionedStorageMethodsExist(t *testing.T) {
+	// If DoltStore doesn't implement all VersionedStorage methods,
+	// this file won't compile. This test exists for documentation.
+	t.Log("DoltStore implements all VersionedStorage methods")
+}