fix(gitattributes): pin *.txt to LF

.gitattributes pins LF for every other text format in the repo (*.md,
*.tmpl, *.yml, *.yaml, *.json, *.toml, *.sh, *.ts, extensionless scripts,
even the hash-pinned diagram-render dist files). *.txt is the one text
format left unpinned.

On Windows with core.autocrlf=true, that means the two tracked .txt files
are rewritten to CRLF at checkout and then read as permanently modified:

  gstack/llms.txt                                   +174 bytes
  make-pdf/test/fixtures/combined-gate.expected.txt  +20 bytes

git status is never clean, and /gstack-upgrade's 'git stash' step saves a
phantom stash on every upgrade — one that pops back to an empty diff.

Co-Authored-By: Claude Opus 5 <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.