Upgrade SyndiKit subrepo from 0.3.7 to main branch

[leogdion]

Context

During the monorepo consolidation (#36), SyndiKit was cloned from the 0.3.7 tag instead of main because the main branch's Package@swift-6.1.swift manifest requires macOS 13.0, which conflicts with the project's macOS 12.0 minimum.

What needs to happen

1. Update the SyndiKit subrepo (Packages/BrightDigit/SyndiKit) from the 0.3.7 tag to the main branch
2. Resolve the macOS minimum version conflict — either:
   • Raise the project's minimum to macOS 13.0, or
   • Update SyndiKit's Package@swift-6.1.swift to lower its minimum to macOS 12.0 (already done on the brightdigit-com-260406 branch of SyndiKit, but that's based off 0.3.7)
3. Update ContributeWordPress/Package.swift local path reference to SyndiKit (already correct — uses ../../BrightDigit/SyndiKit)
4. Ensure swift build and swift test pass after the upgrade

Notes

• SyndiKit main replaces brightdigit/XMLCoder (used in 0.3.7) with CoreOffice/XMLCoder — this changes the transitive dependency
• SyndiKit main also adds a swift-testing test dependency which is what drives the macOS 13 requirement in Package@swift-6.1.swift
• The brightdigit-com-260406 tracking branch on SyndiKit will need to be rebased/reset onto main

Acceptance Criteria

• Packages/BrightDigit/SyndiKit tracks main on brightdigit-com-260406
• swift build passes
• swift test passes
• swift package resolve shows no unexpected remote fetches