rename module to sourcecraft.dev/bigbes/sr-ht-compare; depend on sourcecraft sr-ht-core

go.mod: tidy and complete the README

go mod tidy promotes fernet-go and vaughan0/go-ini from indirect to
direct (the cmd entry point imports go-ini and the smoke test imports
fernet) and drops stale transitive checksums for AWS S3/CLI packages that
core-go pulls but compare.sr.ht never imports. The require block is
otherwise unchanged; build and tests stay green.

README: correct the architecture to reflect the go-git in-process backend
(no runtime git shell-out), document the full local development recipe
(config.ini template, key generation, dev-stub, forging a login cookie),
the deployment steps (install, config propagation for nav, DNS,
internal-ipnet, nginx, User=git systemd), and design notes recording the
go-git decision, the 10 MB bundle rationale, and the pending LICENSE.

compare.sr.ht: project foundation — core package, build scaffolding, core-go fork pin

Bootstrap the go.bigb.es/sourcehut-compare service:

- go.mod (go 1.26.4) pinning core-go to the private fork
  git.srht.bigb.es/~bigbes/core-go @ c2c2f3848fa9 via a replace directive;
  go.sum populated by a throwaway smoke build importing core-go
  config/crypto/client/server + chi + logrus (gates API drift from the
  verification baseline fdb3662 to upstream 71b2787).
- core/: pure domain package — sentinel errors, owner/repo/ref validation
  (check-ref-format-style, hostile-input hardened), and the compare-spec
  grammar (ParseCompareSpec, three-dot/two-dot, percent-unescape). Table
  tests at 97% coverage.
- Makefile (all/build/test/css/bundle/run-dev/install), config.example.ini
  documenting the shared keys read in place, contrib/ nginx block and
  systemd unit, README skeleton, .gitignore.

core-go dep deps and go mod tidy are deferred to later phases per plan.