Replace deploy skill Playground encodedSubId computation 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 **Playground `encodedSubId` computation** in the `foundry-agent/deploy` skill (`deploy.md`).

## Candidate description

**Playground `encodedSubId` computation**

The skill builds the Playground link by converting the subscription id into its 16-byte GUID and encoding it as URL-safe base64 without padding — currently a one-line inline Python invocation. This is a pure, branch-free transform. Formalizing it as a small script (with a PowerShell equivalent) removes the inline Python dependency and is trivially testable.

## Affected file and lines

- [`deploy.md` — Display Agent Information: encodedSubId calculation (L314–L318)](https://github.com/microsoft/GitHub-Copilot-for-Azure/blob/164e0cda7b9d75d6e0d64b235d17bd7ff5e2909e/plugin/skills/microsoft-foundry/foundry-agent/deploy/deploy.md#L314-L318)

## 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 by reading deploy.md at the Display Agent Information section, especially lines 314–318, and inspect the existing inline Python transformation. Add bash and PowerShell scripts, link them from the skill with example invocations and explanations, then run the integration tests to verify equivalent encodedSubId output and end-to-end skill completion.

Written by the indexing model from the issue text.