fix: detect untracked files in sync-versioned-docs workflow

[BaiyuScope3]

Problem

The sync workflow was always skipping commits on first run because git diff only detects changes to tracked files. When v2.6-rc/ doesn't exist yet (first sync), the extracted docs are untracked and invisible to git diff.

Solution

Stage files before checking for changes:

- if git diff --quiet v2.6-rc/; then
+ git add v2.6-rc/
+ if git diff --cached --quiet; then