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/build/mod.rs

@@ -1336,9 +1336,9 @@ Provides: virtual-thing (= 2.0), plain-virtual
             assert!(status.success(), "debian/rules {target} failed");
         }
 
-        let ctx = std::sync::Arc::new(crate::context::Context::new(
-            crate::context::ContextConfig::Local,
-        ));
+        let ctx = std::sync::Arc::new(
+            crate::context::Context::new(crate::context::ContextConfig::Local).unwrap(),
+        );
         let native_arch = crate::debian::arch::native().unwrap_or_else(|_| "amd64".into());
         let opts = crate::build::binary::BinaryMetadataOptions {
             profiles,