[ENHANCEMENT] Return partial results from fetchIssueRelationships on mid-batch failure

[c-vigo]

Context

Copilot review on PR #22 (comment) identified that fetchIssueRelationships discards all successfully fetched relationships when a mid-batch error occurs.

The try/catch wraps the entire batch loop, so if batch N fails, results from batches 1..N-1 are thrown away and an empty Map is returned.

Implementation Plan

• Move try/catch from around the entire for loop to inside the loop body in src/index.ts (fetchIssueRelationships, lines ~415-460)
• On "doesn't exist on type" error: break out of the loop (the sub-issues API is unavailable — no point continuing)
• On other errors (transient/network): core.warning per-batch, then continue to the next batch
• Return the accumulated relationships map (partial results) instead of new Map()
• Update tests in src/__tests__/unit/index.test.ts: add a multi-batch test where batch 1 succeeds and batch 2 fails, and assert that partial results from batch 1 are returned
• Rebuild dist via npm run package