feat(protocol): publish schema revision 1 through Buf

[connorcarpenter15]

Summary

• define the current openengine.v1 contract as schema revision 1, the first valid revision
• add canonical SchemaRevision values to the proto so every consumer-generated language binding receives the same revision metadata
• publish the canonical schema through buf.build/openengine/openengine; consumers pin an immutable BSR module commit and own language-specific generation
• remove repository-owned Python/Rust bindings, package generators, package tests, and package/GitHub-release workflows
• remove Control.Drain plus its request, response, state types, and now-unused lifecycle import; retain standalone shutdown-state signals used for externally initiated draining
• standardize tokenizer/model identity, engine handoff profiles, typed client bootstrap, multimodal routing discovery, and the reduced inference/control/LoRA/KV surface
• define ServerInfo.schema_release as the immutable BSR module commit, with an immutable source commit allowed for unpublished builds

Validation

• buf format --diff --exit-code
• buf lint
• buf build
• buf breaking --against '.git#ref=HEAD'
• buf breaking --against '.git#ref=origin/main'
• npx --yes markdownlint-cli2@0.23.0 "**/*.md"
• Lychee v0.24.2 against repository Markdown: 71 links OK, 0 errors, 1 excluded
• git diff --check

No new language-package tests were added because OpenEngine no longer owns generated language packages; Buf validation is the observable contract boundary for this repository.