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>

docs/consumer-contract.md

@@ -49,10 +49,10 @@ by `flake.lock`. CI runs the _same_ `crucible <build>` as local dev, inside the
 nix devshell (binary-cache fast). No build container, no token, no CI-only
 pre-steps.
 
-The `registry.westgate.pw/deployment/crucible:<sha>` image is **deployment-only**:
-`prod.yml` pins it so tools travel with the runtime host (`nwn.enable`), a
-disabled placeholder until the NWN stack lands. It is **not** a build tool and no
-build/CI job consumes it.
+The `registry.westgate.pw/deployment/crucible:<sha>` image is **retired**
+(2026-07-17): nothing consumed it, `prod.yml` no longer reserves a slot for
+it, and CI no longer builds or publishes it. Binaries, wrappers, and the Nix
+input are the only supported ways to run Crucible.
 
 ## Determinism