Replace microsoft-foundry skill metadata file selection (Step 3) 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 **metadata file selection + environment enumeration (Step 3)** in the `microsoft-foundry` skill (`SKILL.md`, Common Project Context Resolution).

## Candidate description

**Metadata file selection + environment enumeration (Step 3)**

This step applies a fixed file-precedence order to choose a `.foundry/agent-metadata*.yaml` overlay and then reads it to enumerate the available environments and `defaultEnvironment`. The file selection and environment listing are deterministic parsing; only the "prompt the user to choose" branches need agent involvement. A script could list the candidate metadata files and the environments each defines, feeding the agent's selection decision with a compact result instead of inline YAML inspection.

## Affected file and lines

- [`SKILL.md` — Step 3: Select Metadata Overlay and Resolve Environment (L142–L158)](https://github.com/microsoft/GitHub-Copilot-for-Azure/blob/164e0cda7b9d75d6e0d64b235d17bd7ff5e2909e/plugin/skills/microsoft-foundry/SKILL.md#L142-L158)

## 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.

Start with Step 3 in plugin/skills/microsoft-foundry/SKILL.md (L142–L158) and identify the metadata files, environments, and defaultEnvironment the skill needs. Create bash and PowerShell scripts that produce the compact selection data, then update the skill with linked references and examples. Run the integration tests and confirm the skill still completes end-to-end.

Written by the indexing model from the issue text.