internalauth: take both ends of the internal protocol from ecore

The guard on /internal/repos and the header cmd/dolt-git-hook minted for it
were two hand-written halves of one protocol in two packages that shared no
type, no constant and no test. Both are now sr-ht-ecore/internalauth: Guard on
the receiving end, AuthorizationAs on the calling one, over one Auth struct.

The guard also pins the caller, which the old copy did not: core-go only asks
that a token name some client and node, and on an endpoint that provisions a
database for an arbitrary user that means any holder of the network key will
do. The pinned pair lives in core so the mint and the pin cannot drift apart.

The hook test now runs internalauth.Identify — the real receiving end — over
the header the hook produced, so the two ends are checked against each other
rather than against a third copy of the decode.

feat: auto-provision companion Dolt DBs from git.sr.ht pushes

Add a service-to-service path so pushing a git.sr.ht repo creates a matching
Dolt database at ~owner/name, ready before the user's first `dolt push`.

- web: POST /internal/repos, guarded by internal-IP + network-key `Internal`
  auth (not the browser cookie/CSRF). Resolves/mirrors the owner via
  auth.LookupUser, then CreateRepo + InitStore, rolling back the row if the
  store init fails. Idempotent: an existing companion returns 200, a fresh one
  201 — safe to call on every push.
- cmd/dolt-git-hook: the git.sr.ht post-update-script. Delegates every hook
  stage to the stock /usr/bin/git.sr.ht-update-hook unchanged (argv[0], stdin,
  env, exit code preserved; fail-closed if the delegate is missing), then on
  post-update POSTs the companion create and prints a one-time clone notice.
  Best-effort: never fails a push, degrades to a warning on any misconfig.

Tests cover the endpoint (provision/idempotent/rollback/bad-input) and the hook
(signed request round-trips through the guard's decryption, notice only on 201).