fix: use Mosaic credential loader for Gitea API token resolution

The broken `tea login list | awk '{print $NF}'` pattern extracted
the table border character instead of a token, causing pr-metadata.sh
to return empty fields (baseRefName, etc.) and breaking pr-merge.sh.

Centralizes get_remote_host and get_gitea_token in detect-platform.sh
with a 3-tier resolution: Mosaic credential loader (host→service
mapping) → GITEA_TOKEN env var → ~/.git-credentials file.

Removes duplicate local copies from 5 scripts that already source
detect-platform.sh.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tools/git/pr-diff.sh

@@ -68,11 +68,10 @@ elif [[ "$PLATFORM" == "gitea" ]]; then
 
     DIFF_URL="https://${HOST}/api/v1/repos/${OWNER}/${REPO}/pulls/${PR_NUMBER}.diff"
 
-    # Use tea's auth token if available
-    TEA_TOKEN=$(tea login list 2>/dev/null | grep "$HOST" | awk '{print $NF}' || true)
+    GITEA_API_TOKEN=$(get_gitea_token "$HOST" || true)
 
-    if [[ -n "$TEA_TOKEN" ]]; then
-        DIFF_CONTENT=$(curl -sS -H "Authorization: token $TEA_TOKEN" "$DIFF_URL")
+    if [[ -n "$GITEA_API_TOKEN" ]]; then
+        DIFF_CONTENT=$(curl -sS -H "Authorization: token $GITEA_API_TOKEN" "$DIFF_URL")
     else
         DIFF_CONTENT=$(curl -sS "$DIFF_URL")
     fi