Add WORKFLOW_NAME uniqueness documentation to picam secrets example

[Copilot]

Added clarifying documentation to my_secrets_example.py to address issues with identical WORKFLOW_NAME values across different streaming devices.

Problem

The AWS Lambda function uses WORKFLOW_NAME to identify and manage YouTube broadcasts. When multiple devices use identical workflow names, the end_active_broadcasts_for_device() function can terminate the wrong streams since it matches broadcasts by workflow name rather than a unique identifier that includes the camera ID.

This was observed when setting up multiple streams with the same workflow name like [undecided]-[undecided]-SDLT, where ending one stream would terminate others with the same name.

Solution

Added a comprehensive comment block above the WORKFLOW_NAME variable explaining:

• That WORKFLOW_NAME must be unique across all devices and streams
• Why this matters (AWS Lambda function uses the name to identify/manage broadcasts)
• The consequences of identical names (stream conflicts and premature termination)
• Recommendation to keep names short due to potential Lambda processing limitations

Changes

• Added 5 lines of documentation to src/ac_training_lab/picam/my_secrets_example.py
• No functional code changes
• Users will see this guidance when copying the example file to create their my_secrets.py

Fixes #290.