Add ca-certificates feature

[Copilot]

Adds a new ca-certificates devcontainer feature that installs the system CA certificate bundle and optionally registers custom CA certificates from user-provided URLs.

What's added

• src/ca-certificates/ — feature source
  • devcontainer-feature.json — urls option: newline-separated list of certificate URLs to download and register
  • install.sh — installs ca-certificates via apt; if urls is set, downloads each cert to /usr/local/share/ca-certificates/ using wget (preferred) or curl (installed on demand), then runs update-ca-certificates
  • NOTES.md — feature documentation
• test/ca-certificates/test.sh — verifies /etc/ssl/certs/ca-certificates.crt exists and update-ca-certificates is on PATH
• README.md — feature table entry inserted alphabetically between bun.sh and ccache.dev

Notes

• Filename derivation from URL uses tr -cd 'a-zA-Z0-9._-' to prevent path traversal; URLs that yield an empty sanitized filename are skipped with a warning
• Download tool selection: wget if present → curl if present → install curl then use it