feat: Add previous workday mode to Mattermost extractor and enhance sync script

scripts/mattermost/README.md

@@ -60,6 +60,20 @@ bash scripts/mattermost/sync.sh
 
 OpenCode can use this script directly. If `FIDELITY_MATTERMOST_SYNC_CMD` is not set, the workspace plugins will fall back to this wrapper automatically.
 
+Previous workday mode for standups:
+
+```bash
+bash scripts/mattermost/sync.sh --previous-workday
+```
+
+This mode starts from the previous calendar day and expands backward until it finds a day with Mattermost activity in the configured channels. It handles Mondays, weekends, holidays, and OOO gaps without relying on workspace logs.
+
+Useful options:
+
+- `--today YYYY-MM-DD`
+- `--max-lookback-days N`
+- `--output-file PATH`
+
 ## Bootstrap
 
 You can initialize the local runtime with: