Retire the Crucible container image

Nothing consumes registry.westgate.pw/deployment/crucible: prod.yml no
longer reserves a slot for it and no runtime or recovery path pulls it.
Binaries, wrappers, and the Nix input are the supported ways to run
Crucible. Delete the image workflows (build/publish/test), the
Dockerfile, and the make image target; update docs to match.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Makefile

@@ -1,6 +1,6 @@
 SHELL := bash
 .ONESHELL:
-.PHONY: check test vet build smoke fmt image
+.PHONY: check test vet build smoke fmt
 
 # Lint + unit tests. Mirrors the `test` CI job; runs green inside `nix develop`.
 check: vet test
@@ -22,9 +22,3 @@ smoke: build
 
 fmt:
 	gofmt -l -w cmd internal
-
-# Build the Crucible image locally. Requires docker; mirrors build-image CI.
-IMAGE ?= crucible
-GIT_SHA ?= $(shell git rev-parse --short=12 HEAD 2>/dev/null || echo unknown)
-image:
-	docker build --build-arg GIT_SHA=$(GIT_SHA) -f docker/Dockerfile -t $(IMAGE):$(GIT_SHA) .