Phase 1: a working git remote with access control

Phase 1: a working git remote with access control

Vole is now a functioning forge foundation: one CGO-free binary that serves
real git over smart HTTP and enforces per-repo policy in pre-receive.

What is in:

- internal/transport drives `git http-backend` as a CGI subprocess. Chunked
  uploads are spooled to learn CONTENT_LENGTH (git's default client config
  produces them), spool files are cleaned on error and abort, and a janitor
  sweeps anything a crash left behind. Pushes are capped by size and
  serialised behind a semaphore, because Phase 0 measured 1.13 GB of git RSS
  for a single 1 GiB push.
- internal/acl stores roles on refs/vole/acl inside each repository, so
  access rules travel with a clone. One evaluation function is shared by the
  hook and the web layer.
- internal/hook enforces policy in pre-receive: authentication, write role,
  protected refs, per-blob size limits, and a refusal to accept pushes to
  refs/vole/* - without that last check a writer could push their own
  acl.json and grant themselves admin.
- internal/auth holds users, personal access tokens and web sessions in
  SQLite. Passwords use PBKDF2-SHA256; tokens are 256-bit random values and
  are stored as SHA-256 hashes, since there is no low-entropy secret to grind.
- internal/index is the derived read model, rebuildable from disk and refs.
- internal/repo owns the bare-repo store and the metadata ref CAS helper.
- internal/web is the browse surface: repo list, tree, blob, raw, history,
  login, and per-repo settings for access rules, rename and delete.
- internal/signing creates the instance ed25519 key at first boot for the
  Phase 7 work.

Every Phase 1 exit criterion has a test that would fail if it regressed:
clone/push against a real listener, read-only and anonymous rejection with the
ref proven unmoved, rebuild-from-refs compared against the actual acl ref, a
spool file proven absent after an aborted chunked upload, and two pushes
completing through a single-slot semaphore. scripts/ci.sh runs gofmt, vet,
build, a markdown link check and the race-enabled suite.

Verified by hand as well: a clean `vole init` -> user -> token -> push ->
clone cycle, web login and browse, and a 256 MiB chunked push killed
mid-flight leaving no ref, no spool file and no orphaned git processes.