[Feature]: Expose issueNumber in openclaw code run --json output

[zhyongrui]

Summary
Add one stable top-level numeric field to openclaw code run --json named issueNumber.

Problem to solve
Downstream tooling currently has to inspect issue.number directly just to read this nested numeric value. That is awkward for simple JSON consumers.

Proposed solution
Update src/commands/openclawcode.ts so the JSON output includes issueNumber: number | null.

• set it to the nested issue.number value when present
• otherwise emit null

Add or adjust unit tests in src/commands/openclawcode.test.ts to cover both cases.

Impact
Affected users/systems/channels
Tools and scripts that read openclaw code run --json.

Severity
Low.

Frequency
Whenever downstream tooling wants this numeric value without unpacking nested workflow metadata.

Consequence
Without the derived field, simple consumers keep reimplementing the same nested null-check logic.