Give the Expert a tool to toggle plan mode

[andypalmi]

Part of #8364. Sibling of #8510.

Plan mode is user-driven today: the toggle in the Expert chat input (ff-toggle-switch, ExpertChatInput.vue) flips planMode in product-expert.js via setPlanMode(), and context.js sends it to the agent each turn. The Expert cannot enter plan mode itself.

Add a frontend ui_ tool ui_set_plan_mode (like ui_set_onboarding_phase), audience expert, whose handler calls setPlanMode(enabled) with a required enabled boolean argument, the same action the toggle uses. The switch visibly moves and the next turn's context.js reports the new planMode. This is a general Expert improvement, and it gives onboarding its two-phase sequence: provision with plan mode off, then turn it on to build.

Older platforms without the tool: the Expert discovers tools via search and will not call what is not advertised, so it degrades gracefully and the user can still toggle by hand.

Done when

• A frontend audience: expert ui_ tool sets plan mode by calling setPlanMode() with an enabled boolean.
• Enabling or disabling it moves the visible toggle, and the next turn's context reflects it.
• When the tool is not advertised (older platform), the Expert proceeds without it rather than erroring.