[Feature]: Explicit task dependency declarations in tasks.md to enable parallel execution

### Problem Statement

The default `tasks-template.md` lists tasks sequentially with no dependency metadata. Agents cannot determine which tasks are independent and safe to run concurrently.

### Proposed Solution

Add three constructs to the tasks template:

### 1. `(depends on ...)` suffix on each task

```md
- [ ] T001 Create project structure
- [ ] T002 Initialize project with dependencies (depends on T001)
- [ ] T003 Configure linting tools (depends on T001)
```

Tasks with no dependencies omit the suffix. This is **required** for any task that depends on another.

### 2. Dependency DAG section at the bottom of tasks.md

```text
## Dependencies & Execution Order

Wave 1: T001 (no deps)
Wave 2: T002, T003 (T001 done → parallel)
Wave 3: T004, T005, T006 (T002 done → parallel agents)
...
```

### Alternatives Considered

_No response_

### Component

Spec templates (BDD, Testing Strategy, etc.)

### AI Agent (if applicable)

None

### Use Cases

_No response_

### Acceptance Criteria

_No response_

### Additional Context

## Why
- `/implement` can schedule tasks optimally instead of strict sequential execution
- `/taskstoissues` can generate GitHub Issues with "blocked by" references