feat: add herdr dev container feature

[jsburckhardt]

Summary

Adds a new Dev Container Feature for herdr, a CLI tool installed from GitHub releases into /usr/local/bin.

Changes

Created

• src/herdr/devcontainer-feature.json — feature metadata (id, version 1.0.0, version option defaulting to latest)
• src/herdr/install.sh — installer that resolves version via GitHub API, downloads the raw binary asset matching arch/OS, installs to /usr/local/bin/herdr, and verifies with --version
• test/herdr/test.sh — basic smoke test
• test/_global/herdr-specific-version.sh — pinned-version test (v0.7.3)

Updated

• test/_global/all-tools.sh — smoke check for the herdr binary
• test/_global/scenarios.json — added to all-tools.features and new pinned scenario
• .github/workflows/test.yaml — added herdr to the matrix
• README.md — features table row and usage section

Implementation notes

• Downloads raw binary herdr-{linux,macos}-{x86_64,aarch64} (no tarball).
• Arch map: x86_64/amd64/aarch64/arm64. Unsupported arches (i686, armv7l) fail with a clear error — upstream does not publish those assets.
• Version defaults to latest, resolved via /repos/ogulcancelik/herdr/releases/latest.

Validation

devcontainer features test -f herdr -i ubuntu:latest ✅ passes — verified herdr 0.7.3.