feat: Implement Slack export importer and enhance README documentation for historical context recovery

scripts/README.md

@@ -6,6 +6,7 @@ This directory contains helpers that automate:
 - standup generation
 - manager update drafting
 - Mattermost-ready message formatting
+- historical Slack import
 
 The default workspace Mattermost extractor now lives in:
 
@@ -43,3 +44,14 @@ Expected behavior:
 - return a non-zero exit code on failure
 
 OpenCode can then use that output to refresh `ai/inbox/mattermost-latest.md` proactively.
+
+Historical Slack exports can also be imported through:
+
+- `python3 scripts/slack/import_slack_export.py ...`
+- `/slack-import ...`
+
+Recommended raw archive location:
+
+- `archives/slack/export/`
+
+The importer can auto-detect `fidelity*` channels and auto-tune message selection for very large exports.