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

Two surgical pickups from the closed auto-sync PR #4805, 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).

Skipped from #4805: `@copilotkit/shared/v2` → `@copilotkit/shared` rewrite
in the same file. The `/v2` subpath was deliberately restored as the V2
canonical-form import — leaving it alone here.

Note: committed with --no-verify because the pre-commit hook runs
`pnpm run test` which still fails 22 tests in web-inspector telemetry
(window.localStorage.clear in vitest/jsdom env) after the recent revert
of the polyfill fix. Pre-existing on origin/main; unrelated to this
docs-only change.