Remove duo_client_executed_flow_governance feature flag

What does this MR do and why?

Removes the duo_client_executed_flow_governance feature flag, which has been fully rolled out.

The flag governed how Ai::DuoWorkflows::FlowExecutionAuthorizer classifies a flow run: client-executed (agentic chat + flow entitlement) vs. background (AI catalog governance). With the flag always enabled, the classification now runs unconditionally.

Changes:

• Delete the feature flag YAML definition
• Remove feature_flag_enabled? and log_classification from FlowExecutionAuthorizer (the log helper was only needed to measure impact before rollout)
• Collapse client_executed? to contain the body directly, removing the now-redundant client_executable? helper that existed only to separate the flag-gated path
• Remove all stub_feature_flags(duo_client_executed_flow_governance: ...) calls and the "when the feature flag is off" spec context
• Add two paragraphs to the intro of the 'Turn foundational flows on or off' section in doc/user/duo_agent_platform/flows/foundational_flows/_index.md, clarifying that those per-container settings govern only flows GitLab runs on GitLab-managed infrastructure and do not apply to sessions the user runs themselves (such as a GitLab Duo CLI session); this closes the documentation checklist item from the rollout issue

References

#608032 (closed)

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