feat(notsecrets): add multi-provider secret resolution system

Add a config-driven, strongly-typed secret/env resolution system to
notsecrets with 11 provider backends. The system supports explicit
key bindings with typed SecretRef enums and a priority chain fallback.

Tier 1 providers (implemented): Env, 1Password, dotenvx, SOPS, GSM
Tier 2 providers (stubbed): nuenv, direnv, mise, Bitwarden, Vault, dotenvy

New types: Provider, ProviderConfig, SecretRef, SecretsConfig,
SecretsError, SecretSource, EnumerableSecretSource, SecretResolver

Config lives in notsecrets.toml with typed serde deserialization.
ISP split: EnumerableSecretSource for backends that support bulk
enumeration, SecretSource for single-key lookup only.

Includes conformance test suite, 33 new tests (161 workspace total).
notstrap EnvInjector migration deferred to follow-up.

crates/notsecrets/src/sources/dotenvx.rs

@@ -0,0 +1,89 @@
+use crate::config::Provider;
+use crate::error::SecretsError;
+use crate::ports::{EnumerableSecretSource, SecretSource};
+use std::collections::HashMap;
+use std::path::PathBuf;
+use std::process::Command;
+
+pub struct DotenvxSource {
+    env_file: PathBuf,
+}
+
+impl DotenvxSource {
+    pub fn new(env_file: PathBuf) -> Self {
+        Self { env_file }
+    }
+
+    fn decrypt_all(&self) -> Result<HashMap<String, String>, SecretsError> {
+        let output = Command::new("dotenvx")
+            .args([
+                "run",
+                "--env-file",
+                &self.env_file.to_string_lossy(),
+                "--",
+                "env",
+            ])
+            .output()
+            .map_err(|e| SecretsError::SourceError {
+                name: "dotenvx".to_string(),
+                source: e.into(),
+            })?;
+        if !output.status.success() {
+            let stderr = String::from_utf8_lossy(&output.stderr);
+            return Err(SecretsError::SourceError {
+                name: "dotenvx".to_string(),
+                source: anyhow::anyhow!("dotenvx run failed: {stderr}"),
+            });
+        }
+        let stdout = String::from_utf8_lossy(&output.stdout);
+        let mut map = HashMap::new();
+        for line in stdout.lines() {
+            if let Some((k, v)) = line.split_once('=') {
+                let k = k.trim();
+                if !k.is_empty() && !k.starts_with('#') {
+                    map.insert(k.to_string(), v.to_string());
+                }
+            }
+        }
+        Ok(map)
+    }
+}
+
+impl SecretSource for DotenvxSource {
+    fn name(&self) -> &str {
+        "dotenvx"
+    }
+
+    fn provider(&self) -> Provider {
+        Provider::Dotenvx
+    }
+
+    fn resolve(&self, key: &str) -> Result<Option<String>, SecretsError> {
+        self.decrypt_all().map(|m| m.get(key).cloned())
+    }
+}
+
+impl EnumerableSecretSource for DotenvxSource {
+    fn resolve_all(&self) -> Result<HashMap<String, String>, SecretsError> {
+        self.decrypt_all()
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn dotenvx_source_name_and_provider() {
+        let source = DotenvxSource::new("/tmp/test.env".into());
+        assert_eq!(source.name(), "dotenvx");
+        assert_eq!(source.provider(), Provider::Dotenvx);
+    }
+
+    #[test]
+    fn dotenvx_source_resolve_missing_file_returns_error() {
+        let source = DotenvxSource::new("/nonexistent/.env".into());
+        let result = source.resolve_all();
+        assert!(result.is_err());
+    }
+}