docs: consolidate prerequisites into one canonical five-section guide

[brooke-hamilton]

Summary

This is PR 2 (Prerequisites / dev environment) of the Agent Ex — Phase 3 (Contributing docs) plan — see the PR 2 task under §6 of that plan. It makes contributing-code-prerequisites/README.md the single canonical source of truth for setting up a Radius dev environment.

Merge target

⚠️ This PR targets the brooke-hamilton/agentex-phase3 plan branch, not main. It is one of seven stacked Batch A PRs; approving and merging it lands the change on the plan branch only — it does not merge anything to main. The whole phase rolls up to main in a single follow-up PR after all Batch A PRs (and the PR 8/9 index work) land.

Scope

• Canonical prerequisites doc — reformatted contributing-code-prerequisites/README.md into the standard Purpose → Prerequisites → Steps → Verification → Troubleshooting format. Every command, path, and make target was verified against the current repo (make build, make lint, make generate, pnpm -C typespec install, the go.mod tool directives for mockgen/controller-gen, the autorest/oav devDependencies, and the dev container lockfile workflow).
• Codespaces restored — the previously commented-out GitHub Codespaces section is back with a corrected devcontainer_path (.devcontainer/devcontainer.json; the old path .devcontainer/contributor/devcontainer.json no longer exists). The repo ID and basicLinux32gb (8 GB) match the devcontainer's hostRequirements.
• First-commit steps shrunk to links — first-commit-00-prerequisites and first-commit-01-development-tools now carry a short intro plus links to the canonical doc, and the manual "keep in sync" notes are removed. Co-located tutorial content (Launching VS Code, Related links) is preserved.

The planning docs under specs/002-agent-ex/ were not modified.

Co-authored-by: Copilot 223556219+Copilot@users.noreply.github.com

[github-actions]

Dependency Review

✅ No vulnerabilities or license issues or OpenSSF Scorecard issues found.

Scanned Files

None

[Copilot]

Pull request overview

This PR (Batch A / PR 2 in the Agent Ex Phase 3 plan) refactors the Radius contributor onboarding docs so docs/contributing/contributing-code/contributing-code-prerequisites/README.md becomes the canonical, five-section (Purpose → Prerequisites → Steps → Verification → Troubleshooting) guide for dev-environment setup, and the “first commit” tutorial pages link back to it instead of duplicating setup content.

Changes:

• Rewrites the prerequisites guide into a single canonical setup doc covering Codespaces, VS Code dev containers, and local installs, with verification + troubleshooting.
• Restores and corrects the GitHub Codespaces button/link (devcontainer path updated to .devcontainer/devcontainer.json).
• Shrinks the first-commit prerequisites + tools pages to short intros with links back to the canonical prerequisites sections.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 5 comments.

File	Description
docs/contributing/contributing-code/contributing-code-prerequisites/README.md	Converts prerequisites into a canonical five-section guide and adds verification/troubleshooting plus devcontainer lockfile maintenance guidance.
docs/contributing/contributing-code/contributing-code-first-commit/first-commit-01-development-tools/index.md	Removes duplicated editor/tooling setup instructions and links to the canonical prerequisites doc instead.
docs/contributing/contributing-code/contributing-code-first-commit/first-commit-00-prerequisites/index.md	Replaces duplicated prerequisite instructions with links to the canonical prerequisites doc and container-based setup entry points.