fix(margin): Convert margin.sh and settings to LF, pin line endings

margin.sh and settings/*.conf carried CRLF into the repo, so sourcing
them on the SONiC DUT failed outright:

    -bash: $'\r': command not found
    margin.sh: line 126: syntax error near unexpected token `$'{\r''

settings/*.conf was the quieter half of the same bug: margin_init
sources them, and an unquoted assignment such as CB_I2C_CH=6 became
"6\r", so the SWB transport would have driven I2C with a malformed
channel number.

Content is unchanged - the diff is carriage returns only, and bash -n
passes on every converted file.

Add .gitattributes so this cannot come back: *.sh/*.conf/*.py/*.txt/*.md
are pinned to LF, *.ttl stays CRLF (Tera Term runs on Windows only) and
office formats are marked binary.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014RetWKFZFG1ZHcitQAyhwM

.gitattributes

@@ -0,0 +1,29 @@
+# =============================================================================
+# 換行符政策
+# -----------------------------------------------------------------------------
+# 這個 repo 同時被 Windows（Tera Term 端編輯）與 SONiC/Linux（DUT 端執行）使用，
+# 所以必須明確指定，不能靠各機器的 core.autocrlf。
+#
+# 踩過的坑：margin.sh 帶著 CRLF 進 repo，scp 到 DUT 後 bash 直接掛：
+#     -bash: $'\r': command not found
+#     syntax error near unexpected token `$'{\r''
+# settings/*.conf 同樣危險 —— margin_init 是用 source 讀它們的，未加引號的
+# 賦值（例如 CB_I2C_CH=6）會變成 "6\r"，SWB transport 會拿著壞掉的通道號去打 I2C。
+# =============================================================================
+
+# 在 DUT / Linux 上執行或被 source 的，一律 LF
+*.sh    text eol=lf
+*.conf  text eol=lf
+*.py    text eol=lf
+*.txt   text eol=lf
+*.md    text eol=lf
+
+# Tera Term macro 只在 Windows 上跑，維持 CRLF；標成 -text 讓 git 原樣保存，
+# 避免對既有檔案產生大量純換行符的假 diff
+*.ttl   -text
+
+# 二進位，不要做任何轉換
+*.xlsx  binary
+*.docx  binary
+*.pdf   binary
+*.h5    binary