context: fix deadlock and panics when resolving parented contexts

set_current held the config RwLock for writing across make_context,
which for a context with a parent re-entered the same lock through
Context::new's global-manager lookup, deadlocking 'pkh context use'.
Context building is now lock-free by construction: make_context
resolves parent chains against a snapshot map (also rejecting parent
cycles), and neither set_current nor remove_context holds a guard
while building a Context.

A corrupt contexts.json no longer aborts every command at manager
init: load falls back to the default local-only config, backs the
corrupt file up to contexts.json.bak so a later save cannot silently
destroy it, and a dangling current/parent context falls back to local
with an error log instead of panicking.

src/deb/mod.rs

@@ -397,7 +397,7 @@ mod tests {
         log::info!("Successfully pulled package {}", package);
 
         // Create a fresh local context for this test
-        let ctx = Arc::new(Context::new(crate::context::ContextConfig::Local));
+        let ctx = Arc::new(Context::new(crate::context::ContextConfig::Local).unwrap());
 
         // Change directory to the package directory
         let cwd =
@@ -578,7 +578,7 @@ mod tests {
         let temp_dir = tempfile::tempdir().unwrap();
         let pkg_dir = create_indep_cross_test_source(temp_dir.path());
 
-        let ctx = Arc::new(Context::new(crate::context::ContextConfig::Local));
+        let ctx = Arc::new(Context::new(crate::context::ContextConfig::Local).unwrap());
 
         crate::deb::build_binary_package(
             Some("arm64"),