scaffold skill: 'adk is the only template' framing hides that -a/--agent supports third-party add-on templates

**Is your feature request related to a problem? Please describe.**

`--agent`/`-a` on `scaffold create` already accepts a remote Git spec (`org/repo/path@ref`) that layers a third-party add-on's manifest and starter files over the `adk` base template (`scaffold/utils/remote_template.py`'s `parse_agent_spec`). This is documented generically in `references/flags.md` ("or remote Git URL"), but the `google-agents-cli-scaffold` skill's "adk is the only template" framing gives no indication the mechanism exists or how it'd be used for a real integration. A coding agent following the skill today would never discover or suggest it, even though it works correctly.

I ship one of these myself (a Keel resilience add-on for ADK agents: `MisterTK/keel/packaging/agents-cli-template`), and only found the CLI still supported this cleanly by reading the source, not the skill.

**Describe the solution you'd like**

I'd tried opening a PR with the exact doc change, but pull requests appear to be disabled on this repo, so posting the proposed diff here instead in case it's useful to apply directly.

In `skills/google-agents-cli-scaffold/SKILL.md`, right after the existing "`adk` is the only template" blockquote:

```markdown
### Layering a Vendor/Community Add-on

`--agent`/`-a` also accepts a remote Git spec (`org/repo/path@ref`) that layers a
vendor or community add-on's manifest and starter files over the `adk` base
template — a separate mechanism from the architecture choice above, and the
way a third-party library ships opinionated scaffolding (extra dependencies,
starter config) without agents-cli needing to know about it in advance.
Example: `keelrun` (zero-code retry/timeout/breaker/cache policy for ADK
agents) ships one:

\`\`\`bash
agents-cli scaffold create my-agent -a MisterTK/keel/packaging/agents-cli-template
\`\`\`

See `references/flags.md`'s `--agent` row for the full spec syntax.
```

And in `skills/google-agents-cli-scaffold/references/flags.md`, the `--agent` row's description gains one clause: `... or remote Git URL (`org/repo/path@ref` — also used to layer a vendor/community add-on's manifest + starter files over a base template, e.g. `MisterTK/keel/packaging/agents-cli-template`)`.

**Describe alternatives you've considered**

Leaving it undocumented — but that means the mechanism stays effectively invisible to anyone (human or coding agent) relying on the skill rather than reading the CLI's own source.

Read skills/google-agents-cli-scaffold/SKILL.md and references/flags.md, then inspect scaffold/utils/remote_template.py, especially parse_agent_spec, to confirm the remote Git syntax. Done means both skill documents explain that --agent can layer a vendor or community add-on over the adk base template, include the provided example, and point to the full flag syntax.

Written by the indexing model from the issue text.