Dev container: add git-lfs feature to silence post-checkout warning

[achandmsft]

What

Add the git-lfs dev container feature so the binary is on PATH inside Codespaces / Dev Containers.

Why

In any environment where git lfs install --system has been run on the host (which is the case for the default Codespaces image), git copies a post-checkout hook into every new clone. That hook expects git-lfs on PATH. The dev container image (mcr.microsoft.com/devcontainers/python:3.13-bookworm) doesn't ship git-lfs, so every git checkout / git switch / git restore prints:

This repository is configured for Git LFS but 'git-lfs' was not found on your path.
If you no longer wish to use Git LFS, remove this hook by deleting the 'post-checkout'
file in the hooks directory (set by 'core.hookspath'; usually '.git/hooks').

Repo itself does not use LFS (no .gitattributes, no filter.lfs config, no core.hookspath) — the warning is purely cosmetic, but it fires on every git operation and looks like a real problem to new users.

Deleting .git/hooks/post-checkout works once but the hook comes back on every fresh Codespace / container rebuild. Installing the git-lfs binary inside the container is the only fix that survives rebuilds.

How

One new entry under features:

"ghcr.io/devcontainers/features/git-lfs:1": {}

Adds ~5 s to the first container build and a few MB to the image. No runtime impact.

Verification

• N/A — docs / CI-only change (.devcontainer/devcontainer.json only)
• Built the container; git lfs version returns a version; git checkout no longer prints the warning

Checklist

• Mirrored across both infra-bicep and infra-terraform if applicable — N/A
• No secrets in commits, comments, or PR body
• README updated if user-facing behavior changed — N/A (silent quality-of-life fix)
• One logical change per PR