frontend: tighten extraFiles target validation per damocles review

Follow-up to PR #350 review:

1. New assertion: hyperhive.frontend.extraFiles[*].target must be a
   relative path inside the static dir — leading '/' and '..'
   segments rejected at config eval time. Belt-and-braces against
   string-concat-into-paths escapes (the boundary doc flags this
   pattern even though agent.nix goes through operator review).

2. Documented overwrite semantics in the option doc: collision with
   a default-dist path or with a prior entry's target is a hard-fail
   (`refusing to overwrite existing path …`). To override a default
   file, fork `hyperhive.frontend.dist` instead — extraFiles is
   pure additions.

The collision-hard-fail behaviour was already implemented in
`mergedDist` (in commit a19e156); this commit just makes the
contract explicit in the docstring.

Refs #273, addresses damocles' notes on PR #350.

nix/templates/harness-base.nix

@@ -224,10 +224,19 @@
       the bitburner agent's game page at `/bitburner/`).
 
       The default agent UI remains served at `/`; entries here only
-      add new routes and never replace the default. Path collisions
-      with default-dist filenames are a configuration error and
-      will surface as build failures (the merge derivation aborts
-      on overwrite).
+      add new routes and never replace the default. Overwrite
+      semantics are **hard-fail**: if `target` collides with an
+      existing file or directory in the default dist (or with a
+      prior entry's target), the `mergedDist` build aborts with
+      `refusing to overwrite existing path '<target>' in the
+      default dist`. To override a default file, fork the dist via
+      `hyperhive.frontend.dist` instead — `extraFiles` is for
+      pure additions.
+
+      `target` must be a relative path inside the static dir. An
+      assertion rejects leading `/` and `..` segments at config
+      eval time (string-concat-into-paths safety, even though
+      agent.nix goes through operator review before deploy).
     '';
   };
 
@@ -442,6 +451,24 @@
           || lib.hasSuffix ".svg" (toString config.hyperhive.icon);
         message = "hyperhive.icon must point to an .svg file";
       }
+      # hyperhive.frontend.extraFiles[*].target is concatenated into
+      # $out during the mergedDist build. Reject path traversal
+      # (`..` segments) and absolute paths (leading `/`) — both would
+      # let an entry escape the static dir. agent.nix is operator-
+      # reviewed, so this is belt-and-braces, but it's the kind of
+      # mistake that's easy to make and hard to spot during review.
+      {
+        assertion = lib.all (
+          entry:
+          !(lib.hasPrefix "/" entry.target)
+          && !(builtins.any (seg: seg == "..") (lib.splitString "/" entry.target))
+        ) (lib.attrValues config.hyperhive.frontend.extraFiles);
+        message = ''
+          hyperhive.frontend.extraFiles: `target` must be a relative
+          path inside the static dir — no leading `/`, no `..`
+          segments.
+        '';
+      }
     ];
 
     environment.etc."hyperhive/extra-mcp.json".text = builtins.toJSON config.hyperhive.extraMcpServers;