scaffold: dependency reconciliation is silent — no diff shown, unlike enhance/upgrade

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

`scaffold create -a ` reconciles a third-party template's declared `extra_dependencies` against the base `adk` template's current floor at scaffold time ("Ensuring base template 'adk' dependencies..."). I hit this directly: my own remote template (`MisterTK/keel/packaging/agents-cli-template`) had pinned `google-adk[gcp]>=2.0.0,<3.0.0`, and the CLI silently upgraded it to `google-adk[gcp,otel-gcp]>=2.6.0,<3.0.0` plus added three dependencies my manifest never listed (`google-cloud-logging`, `opentelemetry-resourcedetector-gcp`, `gcsfs`) — with no indication in the terminal output that anything had changed from what the template declared.

This is a genuinely useful safety net (it kept a stale third-party template from producing a broken project), but it's invisible. A template author has no way to know their pinned versions are out of date until they diff the generated `pyproject.toml` by hand against their own manifest.

Compare this to `scaffold enhance`/`scaffold upgrade`, which both print a clear summary before applying anything:

```
Dependency changes:
+ Add: google-cloud-aiplatform[evaluation]>=1.156.0
...
✓ Keep (yours): keelrun>=0.2.0
```

**Describe the solution you'd like**

Print the same kind of "Dependency changes" summary during `scaffold create`'s reconciliation step, so a template author (or anyone reading the terminal output) can see exactly what got added/changed versus what the template itself declared — the same UX `enhance`/`upgrade` already have, just applied to the initial `create` path too.

**Describe alternatives you've considered**

Diffing the generated `pyproject.toml` against the template's own manifest by hand after every `scaffold create` run — works, but easy to miss and not something a template consumer would think to do.

Trace the dependency reconciliation performed by `scaffold create`, then compare it with the existing summary behavior in `scaffold enhance` and `scaffold upgrade`. Use the generated `pyproject.toml` and terminal output to verify that additions and changes are reported against the template's declared dependencies.

Written by the indexing model from the issue text.