fix(gitagent.sh): replace node-shebang symlink with bun wrapper

[Copilot]

bun install -g creates a symlink to a .js file with a #!/usr/bin/env node shebang, causing /usr/bin/env node: no such file or directory at runtime since node is not present in the container.

Changes

• src/gitagent.sh/install.sh: After install, resolve the symlink, remove it, and write a bun run shell wrapper in its place — identical pattern to src/dotenv.org/install.sh:

  local gitagentJs
  gitagentJs=$(readlink -f "${binaryTargetFolder}/${binaryName}")
  rm "${binaryTargetFolder}/${binaryName}"
  printf '#!/bin/sh\nexec bun run "%s" "$@"\n' "${gitagentJs}" > "${binaryTargetFolder}/${binaryName}"
  chmod +x "${binaryTargetFolder}/${binaryName}"

• test/gitagent.sh/test.sh: Add gitagent --version check to exercise the binary at test time, not just assert file existence.
• src/gitagent.sh/devcontainer-feature.json + README.md: Bump version 1.0.0 → 1.0.1.