docs: correct what the sibling-args docs promise

The README suggested giving one sibling a list without a flag only the
project reads, so that the sibling would not get a build directory of
its own. When another path loads the same sibling with the full list,
that filter is what gives it the second directory: two builds of one
project, both in the program (#41). It now says to build such a list
once and give it to every sibling. It also says the default is read
when the spec is made, so a spec made at namespace scope, on another
thread or in a script forwards nothing, and that cfg.projectArgs, the
part ApplyStandardArgs did not interpret, is no substitute: it drops
the target and the build type. The comments on ProjectArgs() and on
Configuration::projectArgs say the same.

The spec comment named --local as a flag the default carries down a
chain of siblings. Through a GitProject hop it breaks: the clone has no
checkouts beside it. The GitProject comment said the clone is keyed by
the args; only the Configuration is, and every arg list builds in the
one clone. AGENTS.md now notes that a stale harness gives no symptom
for a changed default member initialiser: project.cpp silently gets the
old default.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

interfaces/Crafter.Build-Clang.cppm

@@ -483,7 +483,9 @@ export namespace Crafter {
         // cfg.files, say), and without it both flag settings share one bin dir
         // and interleave their outputs there. Populated by ApplyStandardArgs;
         // the args it does recognise are excluded, since their effect already
-        // shows up in target/march/mtune/debug/sysroot/type.
+        // shows up in target/march/mtune/debug/sysroot/type. For the same
+        // reason this is not what to hand a sibling project: ProjectArgs() is
+        // every arg, the recognised ones included.
         std::vector<std::string> projectArgs;
         std::vector<Test> tests;
         // Lint rules for `crafter-build lint`. Populate via AddLintRule.

interfaces/Crafter.Build-External.cppm

@@ -93,21 +93,26 @@ export namespace Crafter {
     // remote's default branch HEAD (sloppy but convenient), set branch to
     // track a branch tip, or set commit to pin to an immutable SHA. args are
     // what the dep's CrafterBuildProject receives. They default to this
-    // project's own, so --target=, --march=, --debug and flags of its own
-    // such as --local reach a whole chain of sibling projects without being
-    // passed. Set them to forward something else: a copy without one of this
-    // project's flags, one with an extra --target=, or {} for nothing.
+    // project's own, read when the spec is made, so --target=, --march=,
+    // --debug and the project's own flags reach every sibling down a chain
+    // without being passed. Set them to forward something else: {} for
+    // nothing, or a list built from this project's args, such as one without
+    // a flag only this project reads. Give such a list to every sibling: a
+    // project that two paths load with two different lists is built twice,
+    // and the program ends up with both builds.
     struct GitProjectSpec {
         GitSource source;
         fs::path projectFile = "project.cpp";
         std::vector<std::string> args = ProjectArgs();
     };
 
-    // Clones the spec's git URL into the per-project external cache (keyed by
-    // url+branch+commit+args+projectFile), recursively LoadProject's the
-    // remote's project.cpp, and returns a stable Configuration* you can drop
-    // into cfg.dependencies. The Configuration is owned by an internal cache
-    // for the lifetime of the build; identical specs share one Configuration.
+    // Clones the spec's git URL into the external cache, recursively
+    // LoadProject's the remote's project.cpp, and returns a stable
+    // Configuration* you can drop into cfg.dependencies. The clone is keyed by
+    // url+branch+commit+projectFile alone, so every arg list builds in the
+    // same working tree, each into variant directories of its own. The
+    // Configuration is owned by an internal cache for the lifetime of the
+    // build, keyed by the clone and the args: identical specs share one.
     Configuration* GitProject(const GitProjectSpec& spec);
 
     // Specification for a local sibling crafter-build project (e.g. an