docs(shell-docs): tighten telemetry opt-out copy + useAgent API rename (#4905)

## Summary

Two surgical pickups from a closed auto-sync PR, applied directly to
shell-docs (the post-cutover canonical authoring location).

**1. `telemetry/index.mdx`** — collapse three opt-out paragraphs into a
single tighter sentence, and add the Inspector dev-console to the scope
of what `COPILOTKIT_TELEMETRY_DISABLED` covers.

**2. `snippets/use-agent.mdx`** — three independent improvements:
- `agent.id` → `agent.agentId` (current v2 API field name).
- New `<Callout>` pointing out that `useAgent({ agentId })` is required
when not using CopilotKit Cloud's public access/license key.
- `subscribe()` `useEffect` cleanup gets `[agent]` in the deps array
(exhaustive-deps; prevents stale subscriber after the agent reference
changes).

## What was deliberately skipped

The auto-sync also wanted to rewrite `@copilotkit/shared/v2` →
`@copilotkit/shared` in `use-agent.mdx`. The `/v2` subpath was
deliberately restored as the V2 canonical-form import — left alone here.

## Test plan

- [ ] CI green
- [ ] Spot-check rendered `/telemetry` page locally / on Railway preview
— opt-out paragraph reads cleaner
- [ ] Spot-check rendered `/{any-framework}/use-agent` — Callout renders
inside the `<Steps>` flow, `agent.agentId` displays where the Agent ID
line was, dependency array shows `[agent]`