Refactor reddit-adapter to remove duplicated isRedditPageUrl helper

[gdiab]

Context

Claude re-review on PR #8 flagged a minor duplication in src/adapters/reddit-adapter.ts.

Reference: #8 (comment)

Problem

isRedditPageUrl duplicates existing host-check logic that already exists in src/lib/reddit.ts via isRedditHost.

Proposed change

• Remove the local isRedditPageUrl helper from src/adapters/reddit-adapter.ts.
• Inline URL parsing + isRedditHost call at the usage site, or provide a shared URL-safe wrapper in src/lib/reddit.ts and reuse it.

Acceptance criteria

• No duplicate host-check helper remains in reddit-adapter.ts.
• Behavior stays the same for valid and invalid URL inputs.
• Existing Reddit adapter tests continue to pass.

Priority

Minor / non-blocking follow-up from PR #8.