Add the golib scaffold and the pg module

Stand up the shared library with its first module: the Postgres plumbing
every service currently copy-pastes — the DSN builder, the pool
constructor, and the migration runner arrproxy proved out.

- Add pg.DSNFromEnv, generalising the identical Sprintf builders in
  encapi, artifactapi and forgebot into one prefixed lookup with
  DATABASE_URL passthrough and libpq fallbacks.
- Add pg.New and pg.NewMigrated, which ping before returning so an
  unreachable server fails at startup rather than on the first query.
- Add pg.Migrate, lifting arrproxy's runner verbatim in semantics and
  generalising the hardcoded advisory-lock key to FNV-1a/64 of a
  caller-supplied name and the embedded set to an fs.FS.
- Add pg/pgtest, unifying the encapi and artifactapi testcontainers
  helpers, with SkipIfShort so container-backed tests self-skip on the
  Docker-less Kubernetes runners.
- Add the Makefile, README and pre-commit config, plus test, pre-commit
  and build pipelines on golib-ci.

pg/pg.go

@@ -0,0 +1,18 @@
+// Package pg holds the estate's shared Postgres plumbing: environment-driven
+// DSN construction, pgxpool construction, and the migration runner every
+// service uses to bring its own schema up to date at startup.
+package pg
+
+import (
+	"io"
+	"log/slog"
+)
+
+// logger returns log, or a logger that discards everything when log is nil, so
+// callers may leave the option unset.
+func logger(log *slog.Logger) *slog.Logger {
+	if log != nil {
+		return log
+	}
+	return slog.New(slog.NewTextHandler(io.Discard, nil))
+}