GH-66: docs: add three standalone usage examples (polling, board status, webhooks)

[alekspetrov]

Summary

Automated PR created by Pilot for task GH-66.

Closes #66

Changes

GitHub Issue #66: Add three standalone usage examples (polling, board status, webhooks)

Context

The SDK has no usage examples. Add three independent, self-contained markdown examples under examples/. Each example is a separate deliverable with no shared code — implement them as three separate subtasks.

Acceptance

• examples/polling.md exists: a short walkthrough (one code block) showing how to poll issues — configure the client, fetch open issues by label, print number + title
• examples/board-status.md exists: a short walkthrough (one code block) showing how to update a project-board Status field — resolve the project item, set the single-select field
• examples/webhooks.md exists: a short walkthrough (one code block) showing how to receive a webhook — register the handler, verify the signature, react to issues.opened
• No existing source files modified (docs only)
• Each file under ~60 lines, standalone, no cross-references between examples

Implementation

Three independent subtasks, one per example file:

1. Polling example — write examples/polling.md only
2. Board-status example — write examples/board-status.md only
3. Webhooks example — write examples/webhooks.md only

Each subtask touches exactly one new file. No subtask depends on another.