feat: default HTTP API backend to on for dev/beta channels

Turn on the experimental effect-httpapi server backend by default for
dev, beta, and local installations so internal users exercise the new
backend. Stable (prod/latest) installs remain on the legacy hono backend.

OPENCODE_EXPERIMENTAL_HTTPAPI=true/1 still force-enables on stable, and
OPENCODE_EXPERIMENTAL_HTTPAPI=false/0 disables it as an escape hatch for
dev/beta users.

packages/core/src/flag/flag.ts

@@ -1,4 +1,5 @@
 import { Config } from "effect"
+import { InstallationChannel } from "../installation/version"
 
 function truthy(key: string) {
   const value = process.env[key]?.toLowerCase()
@@ -10,6 +11,10 @@ function falsy(key: string) {
   return value === "false" || value === "0"
 }
 
+// Channels that default to the new effect-httpapi server backend. The legacy
+// hono backend remains the default for stable (`prod`/`latest`) installs.
+const HTTPAPI_DEFAULT_ON_CHANNELS = new Set(["dev", "beta", "local"])
+
 function number(key: string) {
   const value = process.env[key]
   if (!value) return undefined
@@ -81,7 +86,14 @@ export const Flag = {
   OPENCODE_STRICT_CONFIG_DEPS: truthy("OPENCODE_STRICT_CONFIG_DEPS"),
 
   OPENCODE_WORKSPACE_ID: process.env["OPENCODE_WORKSPACE_ID"],
-  OPENCODE_EXPERIMENTAL_HTTPAPI: truthy("OPENCODE_EXPERIMENTAL_HTTPAPI"),
+  // Defaults to true on dev/beta/local channels so internal users exercise the
+  // new effect-httpapi server backend. Stable (`prod`/`latest`) installs stay
+  // on the legacy hono backend until the rollout is complete. An explicit env
+  // var ("true"/"1" or "false"/"0") always wins, providing an opt-in for
+  // stable users and an escape hatch for dev/beta users.
+  OPENCODE_EXPERIMENTAL_HTTPAPI:
+    truthy("OPENCODE_EXPERIMENTAL_HTTPAPI") ||
+    (!falsy("OPENCODE_EXPERIMENTAL_HTTPAPI") && HTTPAPI_DEFAULT_ON_CHANNELS.has(InstallationChannel)),
   OPENCODE_EXPERIMENTAL_WORKSPACES: OPENCODE_EXPERIMENTAL || truthy("OPENCODE_EXPERIMENTAL_WORKSPACES"),
 
   // Evaluated at access time (not module load) because tests, the CLI, and