fix(agent-skill): refuse a symlinked reference folder inside the skill copy

Install and uninstall checked only the copy and its parent for symlinks, so a
`reference` folder that is a link carried every write and unlink of
`reference/<name>.md` to wherever it pointed. Measured: install overwrote a
`history.md` outside the workspace and uninstall deleted it. A cloned
repository can commit that link, and a Docker case's container can plant it
in the bind-mounted workspace the host injects into. With the skill on by
default, both now ran without the user asking.

Both paths now return 'symlink' for a linked folder inside the copy, before
the first write or unlink, and the refusal messages say so.

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

src/cli.ts

@@ -250,7 +250,7 @@ function describeSkillResult(result: AgentSkillApplyResult, target: string): { o
     },
     symlink: {
       ok: false,
-      text: `${target} (or its parent) is a symlink, refusing to write through it.`,
+      text: `${target}, its parent or its reference folder is a symlink, refusing to write through it.`,
     },
   };
   return messages[result];
@@ -320,7 +320,11 @@ skillCmd
         if (result === 'symlink') {
           // A shared-skills layout (`.agents/skills -> ../.claude/skills`) is deliberate:
           // skip it rather than fail the whole sweep.
-          console.log(palette.warn(`- Skipped ${target}: it (or its parent) is a symlink, so it was not touched`));
+          console.log(
+            palette.warn(
+              `- Skipped ${target}: it, its parent or its reference folder is a symlink, so it was not touched`
+            )
+          );
           continue;
         }
         removedAny = true;