#695 Extension dev workflow doc recommends `deno check` but deno isn't bundled or detected

Description

The extension development guide's "Shared Development Workflow" step 2 says:

Author the extension file — use the Quick Start above; `deno check`.

Running that exact command against a freshly authored extension model fails because `deno` is not installed or bundled with swamp:

``` $ deno check extensions/models/my_model.ts command not found: deno

(exit code 127)

Swamp clearly bundles/uses Deno internally already — extension models are loaded, bundled, and executed via a Deno-based runtime (see \`.swamp/bundles/\` output from \`swamp doctor extensions\`). But there's no CLI-level type-check command that uses that bundled runtime, and no detection/warning when the user follows the doc's literal \`deno check\` instruction on a system without a standalone Deno install.

## Steps to Reproduce

1. Do not have \`deno\` installed as a standalone CLI binary.
2. Follow the extension authoring guide and author a model under \`extensions/models/\`.
3. Run \`deno check extensions/models/<file>.ts\` as instructed by the guide.
4. Observe \`command not found: deno\` (exit 127) instead of a useful type-check result or a clear message that Deno is missing.

## Expected Behavior

One of:
- Swamp exposes its own command (e.g. \`swamp extension check <file>\`) that type-checks using the Deno runtime it already bundles internally, so users don't need a separate Deno install.
- Or, if a standalone Deno CLI really is required for this step, the guide/CLI should detect its absence up front and either skip the step with a warning, or print actionable install guidance instead of leaving the user with a bare \"command not found\".

## Affected Component

Extension model development workflow — guide step 2 in \`references/extension/guide.md\` (\"Shared Development Workflow\"), and/or the extension loader/bundler that already depends on Deno internally.

## Environment

- swamp CLI version: 20260619.001856.0-sha.a7f33798
- OS: macOS (darwin)
- \`deno\` binary: not present on PATH