Retire the Crucible container image (#41)

## Summary

The `registry.westgate.pw/deployment/crucible` image has no consumer: `prod.yml` no longer reserves a slot for it (contract settled in sow-platform PR #64) and no runtime or recovery path pulls it. Binaries, wrappers, and the Nix input remain the supported ways to run Crucible.

## What changed

- Deleted `.gitea/workflows/build-image.yml`, `publish-image.yml`, `test-image.yml`
- Deleted `docker/Dockerfile` and the `make image` target
- Updated README, AGENTS.md, and `docs/consumer-contract.md` to state the image is retired

Existing `deployment/crucible` images in the registry can be deleted at leisure; nothing references them.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Reviewed-on: #41
Co-authored-by: vickydotbat <vickydotbat@tutamail.com>
Co-committed-by: vickydotbat <vickydotbat@tutamail.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) .