Release hygiene: publish images with each release and state which Substrate release AX works with #487

Problem

Getting a working AX install today means building images and pairing versions by reading both codebases:

• No ax-server or ax-task-runner images are published with releases. The manifest references ax-server with a ko:// path, and the default task image isn't public (The default task-runner image is not public #424).
• Nothing states which Substrate release an AX commit works with. AX's latest release is v0.3.1, main has since moved to the Substrate v0.4 API (fix(substrate): adapt to Substrate v0.4 API and harden actor RPCs #457) against an untagged Substrate commit, and chore(substrate): bump github.com/agent-substrate/substrate to v0.5.0 #484 moves to v0.5.0. Pairing them wrong fails with a misleading error (Task creation fails on Substrate 6a35150 and later: AX still sends the removed snapshot_config.on_resume #443, Custom ActorTemplate creation errors are swallowed; Task fails with misleading 'actor template not found' (e.g. tag-referenced image) #366).
• go install github.com/google/ax/cmd/ax@latest installs the v0.3.1 CLI, which calls an RPC the server on main no longer has (Replace UpdateTask with CreateTask and enforce immutability #398).

Ask

• Publish ax-server and ax-task-runner images for each release, referenced by digest in the install manifests.
• Add a compatibility table to the installation docs (linked from the README and CONTRIBUTING): AX release to Substrate release.
• Cut AX releases against Substrate releases, and tag a release whenever the CLI and server stop matching.