Grok provider: plan mode via /plan text mapping

[EnzoTironi]

Before submitting

• I searched existing issues and did not find a duplicate.
• I am describing a concrete problem or use case, not just a vague idea.

Area

apps/server

Problem or use case

Live Grok 0.2.x does not advertise ACP session modes. T3 Plan/Build toggle is a no-op on Grok unless prompt text carries /plan.

Proposed solution

Map interactionMode plan → /plan text on send; show composer interaction-mode toggle for Grok.

Why this matters

Plan mode is a core Grok Build workflow. Without entry mapping, users must type /plan manually or leave T3.

Smallest useful scope

Plan entry only. Plan surface (exit_plan_mode proposed cards, plan.md streaming) is #5409 / closed #4233.

Alternatives considered

Wait for Grok to advertise session modes over ACP — not available on live 0.2.x.

Risks or tradeoffs

Attachment-only prompts need bare /plan; empty-text early return is a bug to fix.

Examples or references

PR #5424. Complementary: #5409.

[ahmed-besic]

I see what you mean, you want an indicator. Right now Grok can enter plan mode if you just tell it to. There is a problem with plan mode now where it would enter plan mode if he feels like he needs to and just get stuck on it. My PR (#5409) does fix that (Hopefully it gets merged). But yours is a specific indicator/button for that. I might look into it and fix that also.