Add devcontainer for contributor environment

[kubeboiii]

What this PR does:

Add a .devcontainer/ setup for GitHub Codespaces and VS Code Dev Containers:

• devcontainer.json on Ubuntu 24.04 with Go 1.26.2, Node 20.19.0, kubectl v1.32.2, Helm 3.8.2, and Docker-outside-of-Docker (host socket, not DinD)
• post-create.sh installs arch-aware kind v0.27.0, enables Yarn via corepack, and runs make update/go-deps and make update/web-deps
• devcontainer-lock.json pins devcontainer feature digests
• .devcontainer/README.md with setup, verification, optional cluster flow, and known limitations
• CONTRIBUTING.md updated with dev container instructions and a fix for the local registry namespace note (pipecd namespace comes from make run/pipecd, not make up/local-cluster)

Verified on VS Code Dev Containers with OrbStack (arm64): container rebuild, docker info, go version, make build/go, docker push localhost:5001/hello-world:test. make up/local-cluster failed in that environment (documented in .devcontainer/README.md). GitHub Codespaces and make check were not run yet.

Why we need it:

CONTRIBUTING.md already points contributors to Codespaces, but no dev container configuration existed. New contributors still install Go, Node, Yarn, Docker, kubectl, kind, and Helm manually. This standardizes the environment and aligns tool versions with CI where applicable.

This supersedes the stub in draft PR #6662, which only had a Dockerfile with Go 1.22 and no devcontainer.json. Scope for v1 is devcontainer/Codespaces only; Gitpod remains deferred.

Which issue(s) this PR fixes:

Fixes #6581

Does this PR introduce a user-facing change?:

• How are users affected by this change: No. This only affects contributor development setup.
• Is this breaking change: No
• How to migrate (if breaking change): N/A