Make dump-manifests recoverable outside the inferred build tree

The backlog sweep found that the user-cited #21-#23 items were already
closed, and the next real pain point was `claw dump-manifests` failing
without a direct way to point at the upstream manifest source. This adds
an explicit `--manifests-dir` path, upgrades the failure messages to say
whether the source root or required files are missing, and updates the
ROADMAP closeout to reflect that #45 is now fixed.

Constraint: Preserve existing dump-manifests behavior when no explicit override is supplied
Rejected: Require CLAUDE_CODE_UPSTREAM for every invocation | breaks existing build-tree workflows and is unnecessarily rigid
Confidence: high
Scope-risk: narrow
Reversibility: clean
Directive: Keep manifest-source override guidance centralized so future error-path edits do not drift
Tested: cargo fmt --all; cargo clippy --workspace --all-targets -- -D warnings; cargo test --workspace; architect review APPROVE
Not-tested: Manual invocation against every legacy env-based manifest lookup layout

rust/crates/rusty-claude-cli/tests/output_format_contract.rs

@@ -174,13 +174,15 @@ fn dump_manifests_and_init_emit_json_when_requested() {
     fs::create_dir_all(&root).expect("temp dir should exist");
 
     let upstream = write_upstream_fixture(&root);
-    let manifests = assert_json_command_with_env(
+    let manifests = assert_json_command(
         &root,
-        &["--output-format", "json", "dump-manifests"],
-        &[(
-            "CLAUDE_CODE_UPSTREAM",
+        &[
+            "--output-format",
+            "json",
+            "dump-manifests",
+            "--manifests-dir",
             upstream.to_str().expect("utf8 upstream"),
-        )],
+        ],
     );
     assert_eq!(manifests["kind"], "dump-manifests");
     assert_eq!(manifests["commands"], 1);