Repair Slack Scoped Historical Holes

1. Spec

• 1.1 Add an OpenSpec change for Slack scoped historical-hole repair.
• 1.2 Validate the change with strict OpenSpec checks.

2. Implementation

• 2.1 Preserve per-channel cursor filtering for normal unscoped Slack runs.
• 2.2 Ignore saved message cursors for messages.resources scoped Slack repair runs.

3. Regression Tests

• 3.1 Cover normal per-channel cursor behavior.
• 3.2 Cover scoped archive rows older than channel_last_ts.

4. Acceptance Checks

• 4.1 Run focused Slack runtime tests.
• 4.2 Run openspec validate repair-slack-scoped-historical-holes --strict.
• 4.3 Live-verify scoped archive keys are retained after deploy and repair rerun.