feat: Add workflow template system for agent-executable checklists

Adds `bd workflow` command group that creates epics with dependent child tasks
from YAML templates. This enables structured multi-step workflows like version
bumps where agents can work through tasks with verification and dependencies.

Commands:
- `bd workflow list` - List available workflow templates
- `bd workflow show <name>` - Show template details and task graph
- `bd workflow create <name> --var key=value` - Instantiate workflow
- `bd workflow status <epic-id>` - Show workflow progress
- `bd workflow verify <task-id>` - Run verification command

Features:
- Variable substitution with {{var}} syntax
- Built-in variables: {{today}}, {{user}}
- Preflight checks before workflow creation
- Hierarchical task IDs under epic (bd-xxx.1, bd-xxx.2)
- Dependency graph between tasks
- Verification commands embedded in task descriptions

Includes built-in `version-bump` template with 21 tasks covering:
- All 10+ version files
- check-versions.sh verification
- Git commit, tag, push
- CI monitoring (goreleaser, npm, pypi, homebrew)
- Local machine upgrades
- Final verification

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

internal/types/workflow.go

@@ -0,0 +1,70 @@
+package types
+
+// WorkflowTemplate represents a workflow definition loaded from YAML
+type WorkflowTemplate struct {
+	SchemaVersion int                `yaml:"schema_version" json:"schema_version"`
+	Name          string             `yaml:"name" json:"name"`
+	Description   string             `yaml:"description" json:"description"`
+	Defaults      WorkflowDefaults   `yaml:"defaults" json:"defaults"`
+	Variables     []WorkflowVariable `yaml:"variables" json:"variables"`
+	Preflight     []PreflightCheck   `yaml:"preflight" json:"preflight"`
+	Epic          WorkflowEpic       `yaml:"epic" json:"epic"`
+	Tasks         []WorkflowTask     `yaml:"tasks" json:"tasks"`
+}
+
+// WorkflowDefaults contains default values for task fields
+type WorkflowDefaults struct {
+	Priority int    `yaml:"priority" json:"priority"`
+	Type     string `yaml:"type" json:"type"`
+}
+
+// WorkflowVariable defines a variable that can be substituted in the template
+type WorkflowVariable struct {
+	Name           string `yaml:"name" json:"name"`
+	Description    string `yaml:"description" json:"description"`
+	Required       bool   `yaml:"required" json:"required"`
+	Pattern        string `yaml:"pattern" json:"pattern"`                 // Optional regex validation
+	DefaultValue   string `yaml:"default" json:"default"`                 // Static default
+	DefaultCommand string `yaml:"default_command" json:"default_command"` // Command to run for default
+}
+
+// PreflightCheck is a check that must pass before workflow creation
+type PreflightCheck struct {
+	Command string `yaml:"command" json:"command"`
+	Message string `yaml:"message" json:"message"`
+}
+
+// WorkflowEpic defines the parent epic for the workflow
+type WorkflowEpic struct {
+	Title       string   `yaml:"title" json:"title"`
+	Description string   `yaml:"description" json:"description"`
+	Priority    int      `yaml:"priority" json:"priority"`
+	Labels      []string `yaml:"labels" json:"labels"`
+}
+
+// WorkflowTask defines a single task in the workflow
+type WorkflowTask struct {
+	ID           string        `yaml:"id" json:"id"`
+	Title        string        `yaml:"title" json:"title"`
+	Description  string        `yaml:"description" json:"description"`
+	Type         string        `yaml:"type" json:"type"`
+	Priority     int           `yaml:"priority" json:"priority"`
+	Estimate     int           `yaml:"estimate" json:"estimate"` // Minutes
+	DependsOn    []string      `yaml:"depends_on" json:"depends_on"`
+	Verification *Verification `yaml:"verification" json:"verification"`
+}
+
+// Verification defines how to verify a task was completed successfully
+type Verification struct {
+	Command      string `yaml:"command" json:"command"`
+	ExpectExit   *int   `yaml:"expect_exit" json:"expect_exit"`
+	ExpectStdout string `yaml:"expect_stdout" json:"expect_stdout"`
+}
+
+// WorkflowInstance represents a created workflow (epic + tasks)
+type WorkflowInstance struct {
+	EpicID       string            `json:"epic_id"`
+	TemplateName string            `json:"template_name"`
+	Variables    map[string]string `json:"variables"`
+	TaskMap      map[string]string `json:"task_map"` // template task ID -> actual issue ID
+}