Remove unused field from WatchedPRInfo

[SeanMcTex]

The timestamp field in WatchedPRInfo is persisted to UserDefaults but never read anywhere in the codebase. getWatchedStatus(for:) exists but has zero callers.

This became more apparent in PR #28, which added lastUpdatedAt and inadvertently resets timestamp on more conditions than intended.

Tasks:

• Remove timestamp from WatchedPRInfo
• Remove related serialization/deserialization in save()/load()
• Remove the getWatchedStatus(for:) method (also unused)
• Clean up any related test data if applicable