Drop unneeded fetch-depth: 0 from two workflows (#9489)

## Summary

Two workflow checkouts request the full repo history via `fetch-depth:
0` but never use it. Falling back to the default shallow clone speeds
them up at no behavioral cost.

## Analysis

I scanned every `fetch-depth: 0` site in `.github/`:

| Site | Why set | Needed? |
|---|---|---|
| `check_approvals.yml:19` | `git merge-base` + `git diff` between
base/head | **Yes** — merge-base across long-lived branches needs deep
history. Already mitigated with `filter: blob:none`. |
| `create_release.yml:1623` | Feeds `warpdotdev/generate-changelog` |
**Yes** — changelog walks tag history. |
| `feature_flag_cleanup.yml:30` (analyze) | Agent prompt instructs `git
blame` of `Cargo.toml` | **Yes** — blame needs full history. |
| `feature_flag_cleanup.yml:161` (cleanup) | None — agent prompt only
says "clean up references" / "remove from enum" / "modify code" | **No —
removed** |
| `feature_flag_cleanup.yml:218` (create_pr) | Applies a patch then runs
`peter-evans/create-pull-request` | Probably no, but kept conservative —
`peter-evans` only requires deep history when it must include prior
local commits. Worth a follow-up. |
| `delete_release.yml:95` | Job only does `git push
HEAD:refs/heads/<new>` then `git push --delete <old>` | **No — removed**
|

## Changes

- `feature_flag_cleanup.yml` — removed `fetch-depth: 0` from the cleanup
job. The Oz agent edits source files at HEAD; no git history operations
occur in the prompt.
- `delete_release.yml` — removed `fetch-depth: 0` from the delete job.
Renaming a branch is two server-side pushes; only the tip commit is
needed.

## Test plan

- [ ] CI passes on this PR.
- [ ] Next nightly run of `feature_flag_cleanup.yml` succeeds end-to-end
(analyze → cleanup → create_pr).
- [ ] Next manual `delete_release.yml` invocation succeeds (rename +
delete).

.github/workflows/delete_release.yml

@@ -92,7 +92,6 @@ jobs:
         uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
         with:
           ref: ${{ inputs.branch }}
-          fetch-depth: 0
 
       - name: Rename branch to deleted/
         env: