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.

.pre-commit-config.yaml

@@ -0,0 +1,27 @@
+repos:
+  - repo: https://github.com/pre-commit/pre-commit-hooks
+    rev: v5.0.0
+    hooks:
+      - id: trailing-whitespace
+      - id: end-of-file-fixer
+      - id: check-yaml
+      - id: check-added-large-files
+      - id: check-merge-conflict
+
+  - repo: https://github.com/dnephin/pre-commit-golang
+    rev: v0.5.1
+    hooks:
+      - id: go-fmt
+      - id: go-mod-tidy
+
+  # golib has no root-level Go files (everything lives under a module
+  # directory), so the dnephin go-vet hook, which runs `go vet` at the repo
+  # root, fails with "no Go files". Vet the whole module instead.
+  - repo: local
+    hooks:
+      - id: go-vet
+        name: go vet
+        entry: go vet ./...
+        language: system
+        types: [go]
+        pass_filenames: false