[BUG] syncSubIssues default parameter does not match action.yml

[c-vigo]

Context

Copilot review on PR #22 (comment) identified a mismatch between the syncSubIssues default in the function signature and the action input default.

• action.yml sets sync-sub-issues default to 'true' (line 47)
• The run() function correctly parses it as true (line 131)
• But syncIssuesToMarkdown signature defaults syncSubIssues to false (line 215)

The caller always passes the value explicitly, so there is no runtime impact today. However, the function signature should be consistent with the action definition for defensive correctness.

Implementation Plan

• In src/index.ts line 215: change syncSubIssues = false to syncSubIssues = true
• Rebuild dist via npm run package
• No test changes needed (caller always passes explicitly; this is a consistency fix)