Fix source messages silently dropping updates from mutable sources

[groeneai]

Summary

• GitHub notifications are mutable — when someone @mentions you on a PR you authored, GitHub updates the existing notification's reason from "author" to "mention" (same notification ID)
• The previous INSERT OR IGNORE silently dropped these updates since the (source, id) primary key already existed, making mentions/tags invisible to consumer cursors
• Now detects when an existing record's metadata or content has changed, deletes the old row, and re-inserts with a new rowid — so consumers see it as a new unread message
• Unchanged records are still silently skipped (no extra queries for the common case of no changes)

Problem

The source_messages table uses PRIMARY KEY (source, id). GitHub notification IDs are stable per thread — when a notification's reason changes (e.g. author → mention), the API returns the same ID with updated fields. INSERT OR IGNORE skipped these entirely.

Before fix: 55 stored messages — 51 author, 3 comment, 1 assign, 0 mention
Live GitHub API: 10 mention notifications (same IDs as stored author records)

Implementation

In insert_source_messages():

1. Before inserting, check if (source, id) already exists
2. If it exists with identical metadata + content → skip (current behavior, no regression)
3. If it exists with different metadata or content → DELETE old row, then INSERT new one
4. The new row gets a higher rowid, making it visible to consumer cursors that already read the old version

Test plan

• All 326 existing tests pass
• Deployed and verified: Nerve restarted, GitHub source sync registered, next run scheduled
• Verified via DB inspection that the 10 "mention" notifications will be re-inserted on next sync cycle

🤖 Generated with Claude Code