Make Duo Chat goal mode start the Developer flow

What does this MR do and why?

Goal mode used to be cosmetic. Typing /goal showed a token, changed the placeholder, and removed the /goal text, but the message was then sent as an ordinary chat turn. Nothing actually ran a goal.

This MR makes goal mode real. Sending a prompt in goal mode starts the Developer foundational flow in its goal configuration. Duo chat only enables the /goal functionality if the Developer foundational flow is also enabled at a group and/or instance level.

A few design decisions worth noting:

• Goal is a configuration of the existing Developer foundational flow (developer/v1), not a flow of its own. The turn names a mode so the backend picks that configuration, and the flow's identity stays developer/v1, which is what billing and enablement key off.
• Availability is one question asked in one place: a shared getGoalFlow lookup collapses the feature flag, project, flow configuration, and user permission into a single yes/no. The slash command and the + menu row are gated by the same answer, and the query is cache-first so the two callers cost one request.
• Commands and actions are now separate concepts. A slash command is a trigger; START_GOAL is the effect. Typing /goal and clicking the + menu row both run that one action, so the two entry points can't drift apart.
• The prompt object now carries which flow goal mode runs, instead of a plain on/off boolean, so the send path has what it needs.

What is not included in this MR?

This MR doesn't implement a UI to identify that a user message starts a goal or that a session is a goal session in the sessions pill or the session widget:

This depends on two MRs to actually run goals:

• !255291 (closed) adds the mode parameter to the flow-start API.
• gitlab-org/modelops/applied-ml/code-suggestions/ai-assist!6945 (closed) passes mode through the Duo Workflow Service.

References

Epic: gitlab-org#23107

Screenshots or screen recordings

From slash command	From prompt actions menu

How to set up and validate locally

1. Enable the flag: Feature.enable(:duo_chat_goal_command)
2. Enable the Developer flow for the project.
3. Type /goal in Duo Chat, or pick Goal from the + menu. Either way the goal token appears and the /goal text is removed.
4. Send a goal. Confirm the session runs the Developer flow in its goal configuration rather than answering as chat.
5. Turn the Developer flow off for the project and confirm /goal and the Goal row both disappear.

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