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.

foundation: module, core domain, schema, spike-verified dolt interop

Phase 0 foundation for dolt.sr.ht (pure-Go Dolt hosting for SourceHut).

Dependency pins:
- core-go v0.0.0-20260520082310-fdb3662452dc: the production instance's
  submodule commit (fdb3662); must never be upgraded so token/config/crypto
  behavior matches the rest of the instance.
- dolthub/dolt/go v0.40.5-0.20260626152440-45335d44ad79: pseudo-version at the
  commit tagged v2.1.10 (45335d44), matching the installed dolt CLI. The /go
  submodule's latest tag (v0.40.4, 2021) does not interop with a modern CLI;
  matching the CLI commit guarantees a common NBS format (Format_DOLT) and
  remotesapi proto.
- gopkg.in/go-jose/go-jose.v2 v2.6.3: same JOSE version dolt's creds package
  signs EdDSA keypair JWTs with, avoiding a duplicate JOSE lib.
- grpc v1.79.3, logrus v1.8.3, lib/pq v1.10.9, chi/v5 v5.3.1, brant v0.5.1.

Contents:
- core/ pure domain: models, ValidateName/ParseRepoPath, the access matrix
  (Allowed + NotFoundForPrivate), full table-driven tests.
- schema.sql + migrations/0001_initial.sql (brant format).
- config.example.ini, contrib/dolt.sr.ht.conf (nginx), Makefile, scss/main.scss,
  static/logo.svg, README, .gitignore.
- internal/smoke: throwaway import-and-build check (deleted in Phase 3).

Spike (storage/spike_test.go, tag `spike`): bare NBS store via WriteEmptyRepo,
served by remotesrv on an ephemeral single port, round-tripped through the real
dolt CLI v2.1.10 (clone -> insert -> commit -> push -> re-clone -> verify rows).
Green, no version juggling needed beyond selecting the v2.1.10 commit. FS must
be rooted at the repos dir (LocalFilesysWithWorkingDir) so sealed chunk-URL
prefixes are clean relatives.

Build note: dolt pulls go-icu-regex (CGO), which needs ICU4C headers; on this
Mac set CGO_CPPFLAGS/CGO_LDFLAGS to icu4c@78 (documented in README).