[Fix] Brain memories appear recent after historical ingestion

[roomote-roomote]

​Opened on behalf of @mrubens. Follow up by mentioning @roomote-roomote, in the web UI, or in Slack.

What changed

• Date pull-request and GitHub issue memories by merge, close, or creation time instead of mutable remote update timestamps.
• Give canonical and Slack person pages stable source-derived dates so profile refreshes do not make them look newly relevant.
• Replay existing task, pull-request, issue, and person pages once so previously stored effective dates are corrected.
• Scope Slack directory cache reads to the IDs in each paginated API response instead of reloading the whole workspace cache per page.
• Add regression coverage for occurrence precedence, profile anchoring, alias anchoring, and task-memory replay.

Why this change was made

Historical Brain records could appear recent after a backfill or later metadata refresh. That distorted recency-sensitive retrieval even though the underlying work or profile predated ingestion.

Impact

Brain recency now reflects when work actually happened. Historical records are rewritten through bounded, versioned collector replays, and Slack directory backfills keep each cache lookup proportional to the current API page. Living document and meeting date behavior remains unchanged.

[roomote-community]

No new code issues found. See task

• Avoid loading the complete Slack directory cache for every paginated Slack API response.

_{Reviewed 185bc71}