feat: make workflow states dynamic with XState-style statechart config (#147) (#160)

## Summary

Introduces a configurable workflow state machine that replaces all hardcoded
state labels. The default workflow matches current behavior exactly, ensuring
backward compatibility.

## Architecture

### lib/workflow.ts — Core workflow engine

XState-style statechart configuration:

```typescript
type StateConfig = {
  type: 'queue' | 'active' | 'hold' | 'terminal';
  role?: 'dev' | 'qa';
  label: string;
  color: string;
  priority?: number;
  on?: Record<string, TransitionTarget>;
};
```

All behavior is derived from the config:
- Queue states: `type: 'queue'`, grouped by role, ordered by priority
- Active states: `type: 'active'` — worker occupied
- Transitions: defined with optional actions (gitPull, detectPr, closeIssue, reopenIssue)
- Labels and colors: derived from state.label and state.color

### Derivation functions

- `getStateLabels()` — all labels for issue tracker sync
- `getLabelColors()` — label → color mapping
- `getQueueLabels(role)` — queue labels for a role, ordered by priority
- `getActiveLabel(role)` — the active/in-progress label for a role
- `getRevertLabel(role)` — queue label to revert to on failure
- `detectRoleFromLabel()` — detect role from a queue label
- `getCompletionRule(role, result)` — derive transition rule from config

## Files Changed

- **lib/workflow.ts** — NEW: workflow engine and default config
- **lib/providers/provider.ts** — deprecate STATE_LABELS, LABEL_COLORS; derive from workflow
- **lib/providers/github.ts** — use workflow config for label operations
- **lib/providers/gitlab.ts** — use workflow config for label operations
- **lib/services/pipeline.ts** — use getCompletionRule() from workflow
- **lib/services/tick.ts** — use workflow for queue/active labels
- **lib/services/health.ts** — use workflow for active/revert labels
- **lib/tools/work-start.ts** — use workflow for target label

## Backward Compatibility

- DEFAULT_WORKFLOW matches current hardcoded behavior exactly
- Deprecated exports kept for any external consumers
- No breaking changes to tool interfaces or project state

## Future Work

- Load per-project workflow overrides from projects.json
- User-facing config in projects/workflow.json
- Tool schema generation from workflow states

lib/providers/provider.ts

@@ -2,18 +2,37 @@
  * IssueProvider — Abstract interface for issue tracker operations.
  *
  * Implementations: GitHub (gh CLI), GitLab (glab CLI).
+ *
+ * Note: STATE_LABELS and LABEL_COLORS are kept for backward compatibility
+ * but new code should use the workflow config via lib/workflow.ts.
  */
+import { DEFAULT_WORKFLOW, getStateLabels, getLabelColors } from "../workflow.js";
 
-export const STATE_LABELS = [
-  "Planning", "To Do", "Doing", "To Test", "Testing", "Done", "To Improve", "Refining",
-] as const;
+// ---------------------------------------------------------------------------
+// State labels — derived from default workflow for backward compatibility
+// ---------------------------------------------------------------------------
 
-export type StateLabel = (typeof STATE_LABELS)[number];
+/**
+ * @deprecated Use workflow.getStateLabels() instead.
+ * Kept for backward compatibility with existing code.
+ */
+export const STATE_LABELS = getStateLabels(DEFAULT_WORKFLOW) as readonly string[];
 
-export const LABEL_COLORS: Record<StateLabel, string> = {
-  Planning: "#95a5a6", "To Do": "#428bca", Doing: "#f0ad4e", "To Test": "#5bc0de",
-  Testing: "#9b59b6", Done: "#5cb85c", "To Improve": "#d9534f", Refining: "#f39c12",
-};
+/**
+ * StateLabel type — union of all valid state labels.
+ * This remains a string type for flexibility with custom workflows.
+ */
+export type StateLabel = string;
+
+/**
+ * @deprecated Use workflow.getLabelColors() instead.
+ * Kept for backward compatibility with existing code.
+ */
+export const LABEL_COLORS: Record<string, string> = getLabelColors(DEFAULT_WORKFLOW);
+
+// ---------------------------------------------------------------------------
+// Issue types
+// ---------------------------------------------------------------------------
 
 export type Issue = {
   iid: number;
@@ -30,6 +49,10 @@ export type IssueComment = {
   created_at: string;
 };
 
+// ---------------------------------------------------------------------------
+// Provider interface
+// ---------------------------------------------------------------------------
+
 export interface IssueProvider {
   ensureLabel(name: string, color: string): Promise<void>;
   ensureAllStateLabels(): Promise<void>;