Add Beads library API for Go integration

Expose full Storage interface and all types through public beads.go API,
enabling external Go projects (like VC) to import Beads directly instead
of spawning CLI processes.

Changes:
- Expanded beads.go with all public types (Issue, Dependency, Comment, etc.)
- Added all constants (Status, IssueType, DependencyType, EventType)
- Created comprehensive integration tests (beads_integration_test.go)
- Added library usage example at examples/library-usage/
- Documented library integration in README.md

Test coverage: 96.4% on public API, 14 integration tests, all passing.

Closes bd-58, bd-59

Amp-Thread-ID: https://ampcode.com/threads/T-f0093c79-7422-45e2-b0ed-0ddfebc9ffea
Co-authored-by: Amp <amp@ampcode.com>

beads.go

@@ -18,10 +18,21 @@ import (
 
 // Core types for working with issues
 type (
-	Issue      = types.Issue
-	Status     = types.Status
-	IssueType  = types.IssueType
-	WorkFilter = types.WorkFilter
+	Issue          = types.Issue
+	Status         = types.Status
+	IssueType      = types.IssueType
+	Dependency     = types.Dependency
+	DependencyType = types.DependencyType
+	Comment        = types.Comment
+	Event          = types.Event
+	EventType      = types.EventType
+	Label          = types.Label
+	BlockedIssue   = types.BlockedIssue
+	TreeNode       = types.TreeNode
+	Statistics     = types.Statistics
+	IssueFilter    = types.IssueFilter
+	WorkFilter     = types.WorkFilter
+	EpicStatus     = types.EpicStatus
 )
 
 // Status constants
@@ -41,6 +52,29 @@ const (
 	TypeChore   = types.TypeChore
 )
 
+// DependencyType constants
+const (
+	DepBlocks         = types.DepBlocks
+	DepRelated        = types.DepRelated
+	DepParentChild    = types.DepParentChild
+	DepDiscoveredFrom = types.DepDiscoveredFrom
+)
+
+// EventType constants
+const (
+	EventCreated           = types.EventCreated
+	EventUpdated           = types.EventUpdated
+	EventStatusChanged     = types.EventStatusChanged
+	EventCommented         = types.EventCommented
+	EventClosed            = types.EventClosed
+	EventReopened          = types.EventReopened
+	EventDependencyAdded   = types.EventDependencyAdded
+	EventDependencyRemoved = types.EventDependencyRemoved
+	EventLabelAdded        = types.EventLabelAdded
+	EventLabelRemoved      = types.EventLabelRemoved
+	EventCompacted         = types.EventCompacted
+)
+
 // Storage provides the minimal interface for extension orchestration
 type Storage = storage.Storage