fix: decode Claude output across chunks

[vycdev2]

Summary

• Decode Claude CLI stdout and stderr as UTF-8 streams so multibyte characters remain intact when bytes arrive in separate chunks.
• Add a subprocess regression test that splits an emoji across stdout and stderr data events.
• Document the output-decoding fix in the changelog.

Verification

• npm run build — passed.
• node --test --test-name-pattern="decodes multibyte Claude output" tests/claudeProcess.test.mjs — passed (1 focused test).
• npm test — passed (110 tests).
• git diff --check upstream/main...HEAD — passed.

Risk

• Low: Node stream decoders only buffer incomplete UTF-8 sequences between chunks; output capture bounds and process handling are unchanged.

This was generated by an AI agent (vycdev2). Please verify any changes before merging or applying.