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Task: Implement the Sub-groups feature across the application ecosystem in admin.php, timetable.php, coachs-site.php, and dashboard.php.

Functional Logic and Architecture:

Relationship Cardinality: Implement Sub-groups as a many-to-many relationship. An athlete must be able to belong to multiple sub-groups simultaneously without leaving their Main Group.

Creation Flow: Explicitly define the UI flow for creating a sub-group (Name, Description, and Category).

POD Modification: Update the Find POD logic to filter by sub_group_id. If a session in timetable.php is tagged with a sub-group, the rotation must exclude anyone not in that specific ID.

Automations: Create a trigger so that when a new Competition is created, a corresponding Sub-group is automatically generated.

Robustness and Error Handling:

Validation: Implement server-side checks for athlete assignments. Handle edge cases:

Attempting to assign an athlete to a non-existent sub_group_id.

Duplicate assignments (preventing an athlete from being added to the same sub-group twice).

Graceful degradation if a sub-group is deleted but still referenced in a historical timetable entry.

CSV Integration: Update CSV upload/export logic to include a column for Sub-group tags, ensuring it handles comma-separated strings for multiple group memberships.

Files and Impact Zone:

Database: Update schema, likely creating a new sub_groups table and a sub_group_members junction table.

Logic: Modify admin.php for management and coachs-site.php for visibility.

Documentation: Update the index.php How-To-Use page to explain the difference between Main Groups and Sub-groups.

Execution Protocol:

Discovery: Scan the existing database connection and table structures on the Raspberry Pi.

Plan: Provide a specific list of SQL queries and PHP function changes for my approval.

Test: Describe the unit tests you will run to verify that adding an athlete to 'Tuesday Gym' does not remove them from 'Main Group A'.

--- 💾 ACTION REQUIRED: SAVE PROJECT STATE ---
Backup confirmed. Proceeding with AI planning...

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./agent_manager.sh: line 49: gemini: command not found

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ERROR: Plan file is empty or unreadable. Aborting execution.