feat(skill): enable customize-opencode by default, point at full schema

Flips OPENCODE_EXPERIMENTAL_CUSTOMIZE_SKILL on for every channel; set
the env var to false to opt out. The unstable-channel default helper
had no other users, so it goes away.

Adds a 'Full schema reference' section at the top of the skill so the
model knows to fetch https://opencode.ai/config.json for any field the
skill body does not cover, instead of guessing.

Author: kitlangton

packages/core/src/flag/flag.ts

@@ -1,5 +1,4 @@
 import { Config } from "effect"
-import { InstallationChannel } from "../installation/version"
 
 function truthy(key: string) {
   const value = process.env[key]?.toLowerCase()
@@ -11,13 +10,6 @@ function falsy(key: string) {
   return value === "false" || value === "0"
 }
 
-// Channels where new experiments default to ON (unstable / internal users).
-// Stable channels (`prod`, `latest`) stay opt-in.
-const UNSTABLE_CHANNELS = new Set(["dev", "beta", "local"])
-function unstableDefault(key: string) {
-  return truthy(key) || (!falsy(key) && UNSTABLE_CHANNELS.has(InstallationChannel))
-}
-
 function number(key: string) {
   const value = process.env[key]
   if (!value) return undefined
@@ -56,9 +48,9 @@ export const Flag = {
   OPENCODE_DISABLE_CLAUDE_CODE_PROMPT: OPENCODE_DISABLE_CLAUDE_CODE || truthy("OPENCODE_DISABLE_CLAUDE_CODE_PROMPT"),
   OPENCODE_DISABLE_CLAUDE_CODE_SKILLS,
   OPENCODE_DISABLE_EXTERNAL_SKILLS: truthy("OPENCODE_DISABLE_EXTERNAL_SKILLS"),
-  // Default-on for dev/beta/local; opt-in for stable. Set
-  // OPENCODE_EXPERIMENTAL_CUSTOMIZE_SKILL=false to force off, =true to force on.
-  OPENCODE_EXPERIMENTAL_CUSTOMIZE_SKILL: unstableDefault("OPENCODE_EXPERIMENTAL_CUSTOMIZE_SKILL"),
+  // Default-on for every channel. Set OPENCODE_EXPERIMENTAL_CUSTOMIZE_SKILL=false
+  // to opt out.
+  OPENCODE_EXPERIMENTAL_CUSTOMIZE_SKILL: !falsy("OPENCODE_EXPERIMENTAL_CUSTOMIZE_SKILL"),
   OPENCODE_FAKE_VCS: process.env["OPENCODE_FAKE_VCS"],
   OPENCODE_SERVER_PASSWORD: process.env["OPENCODE_SERVER_PASSWORD"],
   OPENCODE_SERVER_USERNAME: process.env["OPENCODE_SERVER_USERNAME"],

packages/opencode/src/skill/prompt/customize-opencode.md

@@ -7,10 +7,22 @@
 # Customizing opencode
 
 opencode validates its own config strictly and refuses to start when a field
-is wrong. The shapes below are the accepted shapes. When in doubt, fetch
-`https://opencode.ai/config.json` (the JSON Schema) and validate against it.
+is wrong. The shapes below cover the common surface area, but they are a
+**summary, not the source of truth**.
 
-Every `opencode.json` should declare `"$schema": "https://opencode.ai/config.json"`
+## Full schema reference
+
+The authoritative list of every config option, with exact field types, enums,
+defaults, and descriptions, lives in the published JSON Schema:
+
+**<https://opencode.ai/config.json>**
+
+If a field is not documented in this skill, or you need to confirm an exact
+shape before writing config, **fetch that URL and read the schema directly**
+rather than guessing. opencode hard-fails on invalid config, so the cost of a
+wrong shape is a broken startup.
+
+Every `opencode.json` should also declare `"$schema": "https://opencode.ai/config.json"`
 so the user's editor catches mistakes as they type.
 
 ## Where files live
@@ -343,7 +355,8 @@ When a user's config is broken and opencode won't start, these env vars help:
 ## When proposing edits
 
 - Validate against the schema before writing. If you are unsure of a field's
-  exact shape, fetch `https://opencode.ai/config.json` rather than guessing.
+  exact shape, or the field is not covered above, fetch
+  `https://opencode.ai/config.json` and read the schema rather than guessing.
 - Preserve `$schema` and any existing fields the user did not ask to change.
 - For agent, skill, and plugin definitions, prefer creating new files in the
   correct location over inlining everything in `opencode.json`.

packages/opencode/test/preload.ts

@@ -36,9 +36,8 @@ process.env["XDG_STATE_HOME"] = path.join(dir, "state")
 process.env["OPENCODE_MODELS_PATH"] = path.join(import.meta.dir, "tool", "fixtures", "models-api.json")
 process.env["OPENCODE_EXPERIMENTAL_EVENT_SYSTEM"] = "true"
 // Tests assert exact skill counts from disk discovery; the built-in
-// customize-opencode skill is opt-in for stable channels and on by default
-// for unstable channels (including "local" where CI runs). Disable it here
-// so disk-discovery tests aren't off-by-one.
+// customize-opencode skill is on by default. Disable it here so
+// disk-discovery tests aren't off-by-one.
 process.env["OPENCODE_EXPERIMENTAL_CUSTOMIZE_SKILL"] = "false"
 
 // Set test home directory to isolate tests from user's actual home directory