Replace azure-prepare skill .NET Aspire detection sequence with a script

## Summary

Copilot has identified a portion of a skill that is a good candidate for replacement with a script.

The candidate is the **.NET Aspire detection sequence** in the `azure-prepare` skill (`plugin/skills/azure-prepare/references/aspire.md`, with duplicated copies in `generate.md` and `scan.md`).

## Candidate description

The skill repeatedly hand-writes the same deterministic detection sequence to decide whether a workspace is a .NET Aspire app and whether it needs special Functions handling:

1. Find the AppHost project — `find . -name "*.AppHost.csproj"`
2. Confirm the Aspire packages — `grep -r "Aspire.Hosting" --include="*.csproj"`
3. Derive the AppHost directory from the project path
4. Scan the AppHost `*.cs` for `ExcludeFromManifest` (informational — local-only resources)
5. Scan for `AddAzureFunctionsProject`, and if present, check whether `AzureWebJobsSecretStorageType` is already configured

This is a strong script candidate because it is:
- **Repeated 3+ times** — written as prose in `scan.md`, as a single bash block in `generate.md`, and as the full dual-shell implementation in `aspire.md`. Each copy is re-derived by the agent and drifts independently.
- **Deterministic** — a fixed sequence of `find`/`grep` file operations with no branching that requires judgment.
- **Output-heavy where little is needed** — the agent only needs a few booleans/paths out of the raw `find`/`grep` output.
- **Error-prone manually** — the AppHost-dir derivation and the `AddAzureFunctionsProject` → `AzureWebJobsSecretStorageType` rule are easy to transcribe incorrectly, and skipping the Functions check is a documented top cause of failed Aspire deployments.

**Sketch — `detect-aspire.{sh,ps1}`:**
- **Input:** workspace root directory (defaults to cwd).
- **Output:** a compact, self-describing result, e.g. `isAspire`, `appHostPath`, `appHostDir`, `hasExcludeFromManifest`, `hasFunctions`, `secretStorageConfigured`, so the agent can branch on the result without parsing raw command output. The remediation decision (whether/how to add `.WithEnvironment("AzureWebJobsSecretStorageType", "Files")`) stays with the agent.

## Affected file and lines

- [`aspire.md` — Step 1 detect (L47–L50)](https://github.com/microsoft/GitHub-Copilot-for-Azure/blob/3890cbfb65c548ce8daa96cabd1d8de63f7bbcca/plugin/skills/azure-prepare/references/aspire.md#L47-L50)
- [`aspire.md` — Step 1a ExcludeFromManifest scan, bash + PowerShell (L57–L70)](https://github.com/microsoft/GitHub-Copilot-for-Azure/blob/3890cbfb65c548ce8daa96cabd1d8de63f7bbcca/plugin/skills/azure-prepare/references/aspire.md#L57-L70)
- [`aspire.md` — Step 4b Functions / AzureWebJobsSecretStorageType scan, bash + PowerShell (L187–L211)](https://github.com/microsoft/GitHub-Copilot-for-Azure/blob/3890cbfb65c548ce8daa96cabd1d8de63f7bbcca/plugin/skills/azure-prepare/references/aspire.md#L187-L211)
- [`generate.md` — "Check for .NET Aspire Projects FIRST" detection block (L9–L15)](https://github.com/microsoft/GitHub-Copilot-for-Azure/blob/3890cbfb65c548ce8daa96cabd1d8de63f7bbcca/plugin/skills/azure-prepare/references/generate.md#L9-L15)
- [`scan.md` — ".NET Aspire Detection" (L57–L69)](https://github.com/microsoft/GitHub-Copilot-for-Azure/blob/3890cbfb65c548ce8daa96cabd1d8de63f7bbcca/plugin/skills/azure-prepare/references/scan.md#L57-L69)

## Next steps

1. **Evaluate the candidate** — confirm the steps are stable and parameterizable, and that the script captures everything the skill needs.
2. **Create both a bash _and_ a PowerShell version** of the script so the skill works across platforms.
3. **Run integration tests** to verify the scripts behave correctly and the skill still completes end-to-end.

## Background Information

### Why replace regular steps with scripts

Replacing a regular, well-defined series of steps with a script can:

- **Reduce token usage** — the skill no longer needs to spell out each command and parse large command output inline; the agent invokes one script and reads a compact result.
- **Improve reliability** — the logic is written and tested once, instead of being re-derived by the agent on every run.
- **Improve determinism** — the same inputs always produce the same steps and output, removing run-to-run variation.
- **Improve speed of execution** — a single script call replaces multiple round-trips of command generation, execution, and large-output parsing.

### Authoring notes for the scripts

- **Reference scripts with markdown links**, not just a bare path to the script file.
- **Include examples** in the skill showing how to run each script (sample invocation with arguments).
- **Briefly explain what each script does** where it is referenced.
- **The script output should explain what it did**, so the agent and user can understand the result without re-inspecting raw command output.