docs + devcontainer: fix scaffolding step, Azure AD rename, and Codespaces launch speedup

[keith-oak]

Summary

Three independent, low-risk improvements that showed up while I was preparing to deliver this workshop. Each is in its own commit so they can be reviewed, split, or dropped individually.

Commit	What	Closes
7c9b2a5	Rename Azure AD → Microsoft Entra ID in English and Japanese workshop intros	#47
7ec45fb	Clarify the Create C# Scaffolding step so it points attendees at .github/copilot-instructions.md (and explains the .github/instructions/ folder is a different mechanism)	#46
e407772	Speed up Codespaces launch for conference wifi — switch .NET install from wget + dotnet-install.sh to the devcontainer feature, drop unused apt packages, pre-warm NuGet via onCreateCommand, and add a prebuild trigger workflow	—

Details

1. Azure AD → Microsoft Entra ID (#47)

Microsoft rebranded Azure AD to Microsoft Entra ID. Four occurrences updated across docs/en/workshop-introduction.md and docs/ja/workshop-introduction.md. Pure docs change, no behaviour impact.

2. Clarify Copilot Instructions step (#46)

The Create C# Scaffolding step currently tells attendees to edit .github/instructions/instructions.md, describing it as an empty pre-created file. In practice:

• .github/instructions/ already ships with three populated files (csharp-migration.instructions.md, python-analysis.instructions.md, testing.instructions.md) — these are glob-scoped via applyTo frontmatter and shouldn't be edited for this step.
• The actual target is .github/copilot-instructions.md at the repo root. It ships pre-populated with Python-focused guidance that attendees need to replace, not append to.

The English and Japanese versions of create-csharp-scaffolding.md are updated to:

1. Point at the correct path (.github/copilot-instructions.md)
2. Say "replace the entire contents" rather than "fill in an empty file"
3. Include an inline admonition explaining the distinction between .github/copilot-instructions.md (single repo-wide) and .github/instructions/ (glob-scoped applyTo files)

3. Codespaces launch speedup

Workshop attendees typically launch the Codespace for the first time on conference or venue wifi, where cold-start download cost is what determines whether they spend their first 10 minutes waiting or exercising. This commit reduces that cost without touching any workshop content:

• .NET via devcontainer feature (ghcr.io/devcontainers/features/dotnet:2) instead of wget + dotnet-install.sh. Features are cached by Codespaces across rebuilds and are baked into prebuild images — the ~200 MB SDK download stops happening on every cold build.
• Drop wget, curl, build-essential from the apt layer. They were there to bootstrap dotnet-install.sh; with the feature move they're unused, and the pinned Python deps (FastAPI, uvicorn, httpx, requests, pytest) install from wheels.
• onCreateCommand: dotnet restore against src/csharp-app-complete so NuGet is warm before the attendee's first dotnet run.
• .github/workflows/codespaces-prebuilds.yml as a trigger/marker workflow — fires on push to main and on devcontainer/requirements changes. The actual prebuild runs on GitHub's Codespaces infrastructure and must also be enabled in Settings → Codespaces → Set up prebuild. Once that's enabled, attendees should see cold-start drop from several minutes to well under a minute.

No changes to attendee-facing files, workshop flow, or reference implementations.

Out of scope (deliberately)

• Please import Japanese instructions in the lab environment for Tokyo #43 (import JA instructions to Tokyo VM) is an ops/delivery request, not a repo change — left for the delivery team.
• Everything in src/csharp-app-complete/ and src/python-app/ is untouched.

Test plan

• Re-read the modified create-csharp-scaffolding.md in both languages end-to-end and confirm the wording still makes sense
• Confirm four Azure AD occurrences in both workshop-introduction.md files are now Microsoft Entra ID
• Launch a Codespace from this branch on a cold image, time the "ready to work" interval, compare to a cold launch from main
• Verify dotnet --version returns 10.0.x inside the Codespace
• mkdocs build against docs/mkdocs.yml to catch any markdown rendering issues from the new admonition
• Enable Codespaces prebuilds in repo settings so the workflow has something to trigger

Thanks to the workshop team for the care that's gone into this repo — it's a genuinely useful piece of material. Happy to split this PR, rework the wording, or drop any commit on request. 🙌

[keith-oak]

Tagging folks who look like the right reviewers based on recent merge history and issue authorship — apologies for the mention noise, feel free to re-route:

• @elbruno — you've merged most of the recent devcontainer and docs PRs; the Codespaces speedup commit is the one most likely to want your eye
• @carlotta94c / @yoshioterada — for the Japanese-side doc changes in commits 7c9b2a5 and 7ec45fb, happy to adjust wording if the phrasing reads oddly
• @PlagueHO — you raised Create C# scaffolding step in Lab does not align to this repo #46 and Lab Refers to Azure AD throughout #47, would appreciate a sanity check that the fixes match what you had in mind
• @liamchampton — given your recent docs passes, a second opinion on the create-csharp-scaffolding.md rewording would be great

Suggested labels if a maintainer can apply them: documentation, enhancement.

I couldn't self-assign reviewers or labels (fork PR from an external contributor), so happy for anyone with maintainer perms to route this as they see fit. 🙏