feat(fd): add fd devcontainer feature

[jsburckhardt]

Summary

Add fd as a new devcontainer feature. fd is a simple, fast and user-friendly alternative to 'find' (43k+ stars, written in Rust).

Changes

New files

• src/fd/devcontainer-feature.json — Feature metadata
• src/fd/install.sh — Installer downloading from GitHub releases
• test/fd/test.sh — Smoke test
• test/_global/fd-specific-version.sh — Version-pinned test (v10.2.0)

Updated files

• test/_global/all-tools.sh — Added fd check
• test/_global/scenarios.json — Added fd to all-tools and version-pinned scenario
• .github/workflows/test.yaml — Added fd to test matrix
• README.md — Added fd to features table and usage section

Closes #96

[jsburckhardt]

Closing - PR was created prematurely. Work is preserved on the branch.