fix(release): fetch file SHAs from base branch, not next-release (#24)

## Why

After #23, the workflow on `robotframework` failed with `CHANGELOG.md write failed after 5 attempts (422): {"message":"repository file already exists [path: CHANGELOG.md]"}`.

Sequence from the log:
- `POST /branches` → 201 (branch created).
- `GET /branches/next-release` → 500, 500, then 200 (the readiness poll worked).
- `fetch_sha` then queried `GET /contents/CHANGELOG.md?ref=next-release` immediately after — that endpoint was still propagating and didn't return the SHA, so `fetch_sha` came back empty.
- `write_file` saw no SHA, used POST (create), and got HTTP 422 because the file genuinely existed on base (and therefore on the freshly forked `next-release`).

Different Gitea API endpoints don't share a single readiness signal for a newly created branch — `/branches` can report ready while `/contents` is still 500/404.

## Changes

- `fetch_sha` now queries `?ref=${BASE_BRANCH}` instead of `?ref=next-release`. Base is stable, and Gitea content writes are content-addressed by blob SHA, so a SHA fetched from main works for PUT on `next-release` (it was just forked from main, so the blob is identical).
- Treat 404 as "file absent, no SHA" and retry other non-200 responses up to 5× with backoff.

## Test plan

- [ ] Trigger Release.yml on `robotframework` (or another repo with an existing CHANGELOG.md on main). Confirm CHANGELOG.md and .version both write via PUT first try, and PR is created.
- [ ] First-release repo (no CHANGELOG.md on main yet): `fetch_sha` returns empty after 404, `write_file` POSTs to create — confirm this path still works.

Reviewed-on: #24

.gitea/workflows/Release.yml

@@ -175,14 +175,29 @@ jobs:
             sleep 2
           done
 
-          # Fetch file blob SHAs from next-release (inherited from base on creation)
+          # Fetch file blob SHA from BASE_BRANCH. next-release was just forked
+          # from base so the blob SHA matches; querying base avoids racing
+          # Gitea's per-endpoint propagation for the new branch (the /contents
+          # endpoint can still 500/404 after /branches reports 200). Returns
+          # empty only when the file genuinely does not exist on base.
           fetch_sha() {
             local path="$1" out meta code body
-            out=$(api_call GET "/contents/${path}?ref=next-release")
+            for i in $(seq 1 5); do
+              out=$(api_call GET "/contents/${path}?ref=${BASE_BRANCH}")
               meta=$(meta_line "${out}"); code="${meta%%|*}"; body=$(body_lines "${out}")
               if [ "${code}" = "200" ]; then
                 printf '%s' "${body}" | jq -r '.sha // empty'
+                return 0
               fi
+              if [ "${code}" = "404" ]; then
+                return 0
+              fi
+              if [ "${i}" = "5" ]; then
+                echo "fetch_sha ${path} failed after 5 attempts (${meta}): ${body}" >&2
+                return 0
+              fi
+              sleep 2
+            done
           }
           CHANGELOG_SHA=$(fetch_sha "CHANGELOG.md")
           VERSION_SHA=$(fetch_sha ".version")