docs(context): add AI SDK usage flow

[EfeDurmaz16]

Summary

Adds a concrete AI SDK usage flow for the Context component.

Fixes #425.

Changes

• Adds an API route example using streamText and returning response usage
• Adds a client page example that passes usage into Context
• Shows ContextTrigger, ContextContent, usage breakdown components, and ContextContentFooter together in a realistic flow

Verification

• pnpm install --frozen-lockfile
• pnpm --filter docs build
• git diff --check

Note: scoped ultracite check for the single MDX file exits with “Expected at least one target file” because the path is ignored by its target rules, so docs build was used as the stronger validation.

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