fix: config loader ignores implicit file errors to avoid reading binary

Viper's config search (SetConfigName + AddConfigPath) could find the
compiled mantle binary instead of mantle.yaml, causing a YAML parse
error. When no --config flag is provided, silently fall back to defaults
on any ReadInConfig error. Explicit --config still hard-errors.

Also updates README Configuration section to clarify that mantle.yaml
is optional with a table of defaults, env vars, and CLI flags.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: michaelmcnees

README.md

@@ -119,7 +119,9 @@ steps:
 
 ## Configuration
 
-Mantle reads configuration from `mantle.yaml`, environment variables, and CLI flags (highest precedence).
+Mantle reads configuration from `mantle.yaml`, environment variables, and CLI flags (highest precedence). A config file is **optional** — if none is found, Mantle uses sensible defaults that work with the docker-compose setup out of the box.
+
+To create a config file, add `mantle.yaml` to your working directory:
 
 ```yaml
 database:
@@ -132,11 +134,17 @@ log:
   level: info
 ```
 
-Environment variables use the `MANTLE_` prefix:
+The values shown above are the defaults. You only need to create this file if you want to override them.
+
+You can also override any setting with environment variables (`MANTLE_` prefix) or CLI flags:
+
+| Setting | Default | Env Var | CLI Flag |
+|---------|---------|---------|----------|
+| Database URL | `postgres://mantle:mantle@localhost:5432/mantle?sslmode=disable` | `MANTLE_DATABASE_URL` | `--database-url` |
+| API address | `:8080` | `MANTLE_API_ADDRESS` | `--api-address` |
+| Log level | `info` | `MANTLE_LOG_LEVEL` | `--log-level` |
 
-- `MANTLE_DATABASE_URL`
-- `MANTLE_API_ADDRESS`
-- `MANTLE_LOG_LEVEL`
+Override precedence (highest to lowest): CLI flags > environment variables > config file > defaults.
 
 ## Architecture
 

internal/config/config.go

@@ -67,10 +67,10 @@ func Load(cmd *cobra.Command) (*Config, error) {
 			// Explicit --config path: hard error
 			return nil, err
 		}
-		// No explicit path: ignore file-not-found, fail on parse errors
-		if _, ok := err.(viper.ConfigFileNotFoundError); !ok {
-			return nil, err
-		}
+		// No explicit path: silently ignore all errors.
+		// Viper may find non-config files matching the name (e.g., the mantle binary)
+		// and fail to parse them. Since no config file was explicitly requested,
+		// falling back to defaults is always safe.
 	}
 
 	// Env vars — explicit binding for nested keys