fix(pi-embed): start the extensions, and refuse a metered run that is not on the gateway model

[pyramation]

Summary

An embedded run loaded its extensions and never started them, so the metered lane silently didn't take effect.

pi emits session_start from session.bindExtensions(...) — a call its own CLI modes make once their UI exists, and createAgentSession never makes. The metered lane selects its model from exactly that event:

// @agentic-kit/pi-ext-metered-model
pi.registerProvider(providerName, config);
pi.on('session_start', async (_e, ctx) => pi.setModel(ctx.modelRegistry.find(providerName, modelId)));

So startRun returned a session on whatever provider key the process happened to hold — outside the gateway, unmetered, and indistinguishable from a healthy run. Verified against the installed pi: the same session reports model: null without the bind and constructive-gateway/deepseek-v4-pro with it.

startRun now fires it, then insists on the result:

const result = await pi.createAgentSession({ ... });
await result.session.bindExtensions({});
assertMeteredModelSelected(run, result.session);   // throws unless
                                                   // model === `${lane.providerName}/${lane.selectedModel}`

The assertion only applies when the metered lane selected a model (selectModel: false and log-only embeddings are unaffected). Under-reported usage is the failure this lane exists to prevent, so a session that ended up elsewhere fails the run rather than billing nothing.

Link to Devin session: https://app.devin.ai/sessions/450ce6d6659c47759c184ae6ec19a2a8
Requested by: @pyramation

[devin-ai-integration]

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