One metadata branch: refs/heads/vole, linear history, audited rewrites

One metadata branch: refs/heads/vole, linear history, audited rewrites

Issues and change requests now share a single branch. Both are human-authored and
low-frequency, and one ref means one push, one fetch and one validation path
instead of per-branch special cases.

  refs/heads/vole
    issues/<ulid>/...
    requests/...        reserved for change requests

Rules, enforced in pre-receive rather than documented and hoped for:

- The tree layout is closed: only issues/ and requests/ may appear, so a typo in
  a directory name is rejected instead of being silently ignored by the web.
- History must be linear. Merge commits on the metadata branch are refused,
  because a merge node says nothing and makes `git log` harder to read than the
  change it describes. Rebasing is what happens instead.
- Force-push requires repo admin and writes a line to the audit log. Rewriting
  discussion history is sometimes necessary (someone pastes a secret) but must
  never be silent, so it is allowed rather than forbidden and recorded.

The CLI no longer fails on a lost race. Because every entity is its own file,
re-applying a change on the new tip cannot conflict, so it re-fetches, redoes the
mutation and retries with jittered backoff. Two things this turned up that a
single-writer test could never have found:

- A lost race has more than one shape. Besides "non-fast-forward" and "fetch
  first", a push can arrive with a stale old-oid ("incorrect old value
  provided") or lose the creation of the branch itself ("reference already
  exists"). All four are retryable; hook rejections are not.
- Retries without jitter keep writers in lockstep: six concurrent writers failed
  five rounds in a row with a deterministic backoff, and converged immediately
  once the sleep was randomised.

Verified by hand: the CLI writes to the vole branch, `git log vole` is a flat
list, and a merge push is refused with both problems reported at once — the merge
node and a malformed issue id in the same tree.