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/versioned.go

@@ -0,0 +1,119 @@
+// Package storage defines the interface for issue storage backends.
+package storage
+
+import (
+	"context"
+	"time"
+
+	"github.com/steveyegge/beads/internal/types"
+)
+
+// VersionedStorage extends Storage with version control capabilities.
+// This interface is implemented by storage backends that support history,
+// branching, and merging (e.g., Dolt).
+//
+// Not all storage backends support versioning. Use IsVersioned() to check
+// if a storage instance supports these operations before calling them.
+type VersionedStorage interface {
+	Storage // Embed base interface
+
+	// History queries
+
+	// History returns the complete version history for an issue.
+	// Results are ordered by commit date, most recent first.
+	History(ctx context.Context, issueID string) ([]*HistoryEntry, error)
+
+	// AsOf returns the state of an issue at a specific commit hash or branch ref.
+	// Returns nil if the issue didn't exist at that point in time.
+	AsOf(ctx context.Context, issueID string, ref string) (*types.Issue, error)
+
+	// Diff returns changes between two commits/branches.
+	// Shows which issues were added, modified, or removed.
+	Diff(ctx context.Context, fromRef, toRef string) ([]*DiffEntry, error)
+
+	// Branch operations
+
+	// Branch creates a new branch from the current state.
+	Branch(ctx context.Context, name string) error
+
+	// Merge merges the specified branch into the current branch.
+	// Returns a list of conflicts if any exist.
+	Merge(ctx context.Context, branch string) ([]Conflict, error)
+
+	// CurrentBranch returns the name of the currently active branch.
+	CurrentBranch(ctx context.Context) (string, error)
+
+	// ListBranches returns the names of all branches.
+	ListBranches(ctx context.Context) ([]string, error)
+
+	// Commit operations
+
+	// Commit creates a new commit with all staged changes.
+	Commit(ctx context.Context, message string) error
+
+	// GetCurrentCommit returns the hash of the current HEAD commit.
+	GetCurrentCommit(ctx context.Context) (string, error)
+
+	// Conflict resolution
+
+	// GetConflicts returns any merge conflicts in the current state.
+	GetConflicts(ctx context.Context) ([]Conflict, error)
+
+	// ResolveConflicts resolves conflicts using the specified strategy.
+	// Strategy must be "ours" or "theirs".
+	ResolveConflicts(ctx context.Context, table string, strategy string) error
+}
+
+// HistoryEntry represents an issue at a specific point in history.
+type HistoryEntry struct {
+	CommitHash string       // The commit hash at this point
+	Committer  string       // Who made the commit
+	CommitDate time.Time    // When the commit was made
+	Issue      *types.Issue // The issue state at that commit
+}
+
+// DiffEntry represents a change between two commits.
+type DiffEntry struct {
+	IssueID  string       // The ID of the affected issue
+	DiffType string       // "added", "modified", or "removed"
+	OldValue *types.Issue // State before (nil for "added")
+	NewValue *types.Issue // State after (nil for "removed")
+}
+
+// Conflict represents a merge conflict.
+type Conflict struct {
+	IssueID     string      // The ID of the conflicting issue
+	Field       string      // Which field has the conflict (empty for table-level)
+	OursValue   interface{} // Value on current branch
+	TheirsValue interface{} // Value on merged branch
+}
+
+// IsVersioned checks if a storage instance supports version control operations.
+// Returns true if the storage implements VersionedStorage.
+//
+// Example usage:
+//
+//	if !storage.IsVersioned(store) {
+//	    return fmt.Errorf("history requires Dolt backend")
+//	}
+//	vs := store.(storage.VersionedStorage)
+//	history, err := vs.History(ctx, issueID)
+func IsVersioned(s Storage) bool {
+	_, ok := s.(VersionedStorage)
+	return ok
+}
+
+// AsVersioned attempts to cast a Storage to VersionedStorage.
+// Returns the VersionedStorage and true if successful, nil and false otherwise.
+//
+// Example usage:
+//
+//	vs, ok := storage.AsVersioned(store)
+//	if !ok {
+//	    return fmt.Errorf("history requires Dolt backend")
+//	}
+//	history, err := vs.History(ctx, issueID)
+func AsVersioned(s Storage) (VersionedStorage, bool) {
+	vs, ok := s.(VersionedStorage)
+	return vs, ok
+}