Revert "research: delete Hono backend (do not merge) (#25667)" and cleanup

This reverts:
- 28b03595b research: delete Hono backend (do not merge) (#25667)
- b24a4e897 chore(server): clean up post-Hono-deletion scar tissue (#26542)

v1.14.42 broke startup for users with plugins that depend on the
Hono wire format (most visibly opencode-gemini-auth, see #26546).
Restoring Hono as the default backend on stable channels while we
investigate the actual plugin compatibility story.

OPENCODE_EXPERIMENTAL_HTTPAPI flag and dual-backend selection come
back. Stable installs default to Hono; dev/beta default to HTTP API.

Conflict resolution: took the pre-deletion side for control-plane
schemas and the seven test files where post-deletion follow-up PRs
had also touched the conflicting lines. The HTTP API code added
since the deletion (compression, cors-vary, fence, lifecycle log,
account error mapping, etc.) is preserved as-is — those still apply
on the HTTP API path for users on dev/beta channels.

packages/sdk/js/script/build.ts

@@ -9,9 +9,16 @@ import path from "path"
 
 import { createClient } from "@hey-api/openapi-ts"
 
+const openapiSource = process.env.OPENCODE_SDK_OPENAPI === "hono" ? "hono" : "httpapi"
 const opencode = path.resolve(dir, "../../opencode")
 
-await $`bun dev generate > ${dir}/openapi.json`.cwd(opencode)
+// `bun dev generate` now derives the spec from the Effect HttpApi contract by
+// default; pass `--hono` to fall back to the legacy Hono spec for parity diffs.
+if (openapiSource === "httpapi") {
+  await $`bun dev generate > ${dir}/openapi.json`.cwd(opencode)
+} else {
+  await $`bun dev generate --hono > ${dir}/openapi.json`.cwd(opencode)
+}
 
 await createClient({
   input: "./openapi.json",