Replace azure-prepare skill azd context detect/apply/verify 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 **azd context detect / apply / verify sequence** in the `azure-prepare` skill (`plugin/skills/azure-prepare/references/azure-context.md`, with duplicated apply/verify copies in `aspire.md`).

## Candidate description

The skill repeatedly hand-writes the same deterministic sequence to read the current Azure context and then apply and verify the user-confirmed subscription and location:

1. **Detect existing context** — `azd env list`, then `azd env get-values` and parse `AZURE_SUBSCRIPTION_ID` / `AZURE_LOCATION`.
2. **Fallback detect** — `azd config get defaults`, then `az account show --query "{name:name, id:id}" -o json`.
3. **Apply** — `azd env set AZURE_SUBSCRIPTION_ID ` then `azd env set AZURE_LOCATION `.
4. **Verify** — re-run `azd env get-values` and confirm the two values took effect.

This is a strong script candidate because it is:
- **Repeated 4 times** — the apply/verify trio appears once in `azure-context.md` Step 6 and three more times in `aspire.md` (initial set, re-set, and troubleshooting fix).
- **Deterministic and order-sensitive** — the docs call out that setting `AZURE_SUBSCRIPTION_ID` immediately (before azd falls back to its own default subscription) is the single most common failure point. A script enforces the ordering every time.
- **Output-heavy where little is needed** — `azd env get-values` dumps many variables but only two matter.
- **Error-prone manually** — re-typing the set→set→verify trio across files invites drift and missed verification.

**Sketch — `set-azd-context.{sh,ps1}`:**
- **Input:** subscription id, location, optional azd env name.
- **Output:** a compact, self-describing result confirming the values that were detected/applied and the post-set verification (the two resolved values), so the agent does not re-parse `azd env get-values`. The interactive confirmation (`ask_user` in `azure-context.md` Step 1) stays with the agent; only the mechanical detect/apply/verify is scripted.

## Affected file and lines

- [`azure-context.md` — Step 1 detect existing (L7–L36)](https://github.com/microsoft/GitHub-Copilot-for-Azure/blob/3890cbfb65c548ce8daa96cabd1d8de63f7bbcca/plugin/skills/azure-prepare/references/azure-context.md#L7-L36)
- [`azure-context.md` — Step 2 fallback detect (L40–L61)](https://github.com/microsoft/GitHub-Copilot-for-Azure/blob/3890cbfb65c548ce8daa96cabd1d8de63f7bbcca/plugin/skills/azure-prepare/references/azure-context.md#L40-L61)
- [`azure-context.md` — Step 6 apply & verify (L143–L179)](https://github.com/microsoft/GitHub-Copilot-for-Azure/blob/3890cbfb65c548ce8daa96cabd1d8de63f7bbcca/plugin/skills/azure-prepare/references/azure-context.md#L143-L179)
- [`aspire.md` — apply/verify trio (L123–L135)](https://github.com/microsoft/GitHub-Copilot-for-Azure/blob/3890cbfb65c548ce8daa96cabd1d8de63f7bbcca/plugin/skills/azure-prepare/references/aspire.md#L123-L135)
- [`aspire.md` — apply/verify trio, repeated (L254–L262)](https://github.com/microsoft/GitHub-Copilot-for-Azure/blob/3890cbfb65c548ce8daa96cabd1d8de63f7bbcca/plugin/skills/azure-prepare/references/aspire.md#L254-L262)
- [`aspire.md` — troubleshooting fix sequence (L401–L411)](https://github.com/microsoft/GitHub-Copilot-for-Azure/blob/3890cbfb65c548ce8daa96cabd1d8de63f7bbcca/plugin/skills/azure-prepare/references/aspire.md#L401-L411)

## 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.