Add template support for issue creation (bd-164b)

- Built-in templates: epic, bug, feature (embedded in binary)
- Custom templates in .beads/templates/ (override built-ins)
- Commands: bd template list/show/create
- Flag: bd create --from-template <name> "Title"
- Template fields: description, type, priority, labels, design, acceptance
- Security: sanitize template names to prevent path traversal
- Flag precedence: explicit flags override template defaults
- Tests: template loading, security, flag precedence
- Docs: commands/template.md and README.md updated

Closes bd-164b

Amp-Thread-ID: https://ampcode.com/threads/T-118fe54f-b112-4f99-a3d9-b7df53fb7284
Co-authored-by: Amp <amp@ampcode.com>

internal/types/types.go

@@ -305,6 +305,8 @@ type WorkFilter struct {
 	Status     Status
 	Priority   *int
 	Assignee   *string
+	Labels     []string   // AND semantics: issue must have ALL these labels
+	LabelsAny  []string   // OR semantics: issue must have AT LEAST ONE of these labels
 	Limit      int
 	SortPolicy SortPolicy
 }