fix: force LF on *.txt so generated llms.txt stays clean on Windows

.gitattributes pins eol=lf for every other generated/parsed text type
(*.md, *.json, *.ts, setup, bin/*) but omits *.txt. gstack/llms.txt is
regenerated as LF by ./setup's gen-llms-txt on every run, while Git for
Windows sets core.autocrlf=true at the system level. The file is therefore
reported modified forever, and /gstack-upgrade's `git stash` silently
stashes it on every upgrade.

Blob and worktree are both pure LF and `git diff` is empty, so the file is
phantom-modified rather than content-modified.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.gitattributes

@@ -8,6 +8,7 @@
 *.yaml       text eol=lf
 *.json       text eol=lf
 *.toml       text eol=lf
+*.txt        text eol=lf
 
 # Bash scripts must always use LF — CRLF in bash scripts produces bizarre
 # "Bad interpreter" / "command not found" errors on Linux runners.