Add lockNode() Pipeline step

### Summary
Add a dedicated Pipeline step `lockNode()` for node-exclusive locking semantics.

### Motivation
Users currently maintain a Jenkins node plus a separate lockable resource with the same name/labels. This is redundant and error-prone. A purpose-built step clarifies intent and improves UX.

### Proposed usage
```groovy
lockNode("agent-1") {
// exclusive use
}

lockNode(label: "hw-s7", quantity: 1) {
node(env.LOCKED_NODE) {
// run
}
}
```

### Behavior (v1)
Provide node-specific environment variables such as `LOCKED_NODE`. Prefer online nodes when selecting candidates.

### Implementation approach (v1)
Start as syntactic sugar over existing locking mechanics (exact mapping to NodesMirror/resources to be decided).

### Acceptance criteria
- Works with node name and node label
- Enforces exclusivity
- Does not require users to manually keep duplicate configuration (or clearly documents what is required)

### Tests
- JenkinsRule pipeline tests for exclusivity + env vars
- Label-based selection tests