fix(route): resume an existing ai/issue-N branch instead of losing re-triggered work

When an agent was re-triggered on an issue that already had a branch/open PR, route.sh created a
fresh branch from main (git checkout -b), so the push back was rejected non-fast-forward and the new
commits were silently dropped (|| true) -- the run reported success and re-announced the stale PR,
but the work was gone (observed on issue #17 run #88). Resume the existing branch when it exists, so
new commits fast-forward onto it and update its PR; only branch from main when none exists.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.gitea/workflows/scripts/route.sh

@@ -55,7 +55,13 @@ if [ -n "$IS_PR" ]; then                     # comment on a PR -> resume its bra
   ref=$(curl -s -H "Authorization: token $GT" "$API/pulls/$NUM" | jq -r .head.ref)
   git fetch origin "$ref" && git checkout "$ref"
   { echo "branch=$ref"; echo "new=false"; } >> "$GITHUB_OUTPUT"
-else                                         # comment on an issue -> new branch
+elif git ls-remote --exit-code --heads origin "ai/issue-$NUM" >/dev/null 2>&1; then
+  # comment on an issue whose branch ALREADY exists (a prior run / open PR) -> RESUME it, so new
+  # commits fast-forward onto the same branch and update its PR. Branching fresh from main here would
+  # be rejected on push as non-fast-forward and the new work would be silently lost (see issue #17).
+  git fetch origin "ai/issue-$NUM" && git checkout "ai/issue-$NUM"
+  { echo "branch=ai/issue-$NUM"; echo "new=false"; } >> "$GITHUB_OUTPUT"
+else                                         # comment on an issue, no branch yet -> new branch
   git checkout -b "ai/issue-$NUM"
   { echo "branch=ai/issue-$NUM"; echo "new=true"; } >> "$GITHUB_OUTPUT"
   # For dev agents, publish the branch immediately and tell the maintainer where to watch.