Allow renaming the launcher without recompilation

[jdevera]

Summary

Right now, if you want to run command-launcher under a different name (say vht instead of cdt), you have to recompile it. This PR makes it so you can just copy or rename the binary and it picks up the new name automatically.

This means you can build a single binary and use it for multiple launchers just by copying it. More importantly, anyone can grab the official release from the GitHub releases page and use it directly for their own launcher. No need to build from source at all.

How it works:

• The app name is taken from the binary's real file path at startup (symlinks are resolved, so a symlink is just an alias: same name, same config)
• If you copy the binary to a new name, it becomes a separate instance with its own config directory
• The long display name (e.g. "Criteo Dev Toolkit") can be set in config via app_long_name, so you don't need to recompile for that either
• If app_long_name isn't set, it falls back to the compiled-in default, so nothing changes for existing users

Example workflow:

cp cdt myapp
myapp config app_long_name "My Cool Launcher"
myapp help  # shows "My Cool Launcher" and "myapp" everywhere

Note on env var prefix: since a copied binary is a new identity, its env var prefix changes too (e.g. MYAPP_ instead of CDT_). This also means dropin scripts that hardcode a specific prefix won't see the right vars under a different binary name. The COLA_ prefixed vars (already exported by the launcher) work regardless of binary name.

Changes

• main.go: resolve the binary path at startup to derive the app name
• cmd/root.go: load config before creating the root command so we can read app_long_name
• internal/config/settings.go: add app_long_name as a string config key
• main_test.go: unit tests for the name resolution logic
• test/integration/test-runtime-name.sh: integration tests for binary name, symlink, copy, and config override
• test/integration/test-cmd-context.sh: updated FULL_COMMAND_NAME test — the old test copied the binary and relied on the copy sharing the original's env var prefix, which is no longer the case. The copy-based name behavior is now covered by test-runtime-name.sh instead.

Tests

• Unit tests pass
• Integration tests pass
• New integration test covers: binary name detection, symlink resolution, copy gets own name, long name from config, original unaffected by copy's config
• Smoke tested manually: build, copy, symlink, config override all work as expected