fix(config): a foreign project must not capture the config path

default_config_path treated any directory holding Cargo.toml plus a
config/ folder as a gateway checkout, so running cortiq-gateway inside
an unrelated project died with "cannot read config config/gateway.toml"
instead of using the per-user home. Now ./config/gateway.toml counts
only when the file is actually readable, the repo-checkout heuristic
requires Cargo.toml to be cortiq-gateway's own, and everything else
falls through to $CORTIQ_GATEWAY_HOME / ~/.cortiq-gateway — typing
cortiq-gateway in any directory picks up the same configuration.

src/config.rs

@@ -924,14 +924,23 @@ impl Default for Config {
 }
 
 /// Where the config lives when `--config` is not given.
-/// Repo checkout (`config/gateway.toml` exists or `config/` dir exists) → keep it there;
-/// installed binary (no repo) → `$data_dir/gateway.toml`.
+/// A readable `./config/gateway.toml` (a real gateway checkout / install dir)
+/// wins; otherwise the per-user home (`$CORTIQ_GATEWAY_HOME` or
+/// `~/.cortiq-gateway`) — so `cortiq-gateway` typed in any directory picks up
+/// the same configuration it was set up with. A foreign project that merely
+/// has `Cargo.toml` and a `config/` folder must NOT capture the config
+/// (that used to shadow the user's real config with a broken path).
 pub fn default_config_path() -> String {
     let repo_path = "config/gateway.toml";
-    if std::path::Path::new(repo_path).exists() {
+    // the file itself, readable — not just an entry that exists()
+    if std::fs::read_to_string(repo_path).is_ok() {
         return repo_path.to_string();
     }
-    if std::path::Path::new("Cargo.toml").exists() && std::path::Path::new("config").exists() {
+    // our own repo checkout before first run: config/ exists, file not yet
+    let ours = std::fs::read_to_string("Cargo.toml")
+        .map(|t| t.contains("name = \"cortiq-gateway\""))
+        .unwrap_or(false);
+    if ours && std::path::Path::new("config").is_dir() {
         return repo_path.to_string();
     }
     data_dir()