Untrack the node_modules symlink and ignore both directory and link forms

Why: 642299d committed node_modules as a symlink pointing at the absolute
path /home/weichen/projects/Agent-System-Dev/modules/frontend/node_modules.
The .gitignore rule "node_modules/" only matches directories, not symlinks,
so the link slipped through and other clones would receive this machine's
path while normal installs get bypassed.

Evidence: git ls-tree HEAD node_modules => mode 120000 blob before this
commit; git check-ignore -v node_modules exits 0 with rule
".gitignore:2:node_modules" after unstaging the link.

Impact: fresh clones no longer carry a machine-specific symlink; local
installs work normally. No code changes.

Refs: code review of feature/chat-process-split-r1 (2026-09-30)

.gitignore

@@ -1,3 +1,4 @@
-node_modules/
+# 无斜杠形式同时忽略目录与符号链接（目录式规则不匹配 symlink，曾致链接被提交）。
+node_modules
 dist/
 *.local

node_modules

@@ -1 +0,0 @@
-/home/weichen/projects/Agent-System-Dev/modules/frontend/node_modules