fix(config): fall back to legacy path when migration fails and use canonical basename everywhere (#3133)

Root cause: loadPluginConfig() unconditionally switched userConfigPath to the
canonical name after calling migrateLegacyConfigFile(), even when migration
failed (e.g. file lock on Windows, permission denied). This left the config
path pointing to a non-existent file, so the plugin config silently loaded
as empty defaults.

Additionally, several fallback/default paths were hardcoded to the legacy
'oh-my-opencode' basename instead of using CONFIG_BASENAME ('oh-my-openagent'),
causing CLI config commands (writeOmoConfig, detectCurrentConfig) to write to
the wrong filename.

Changes:
- plugin-config.ts: check migrateLegacyConfigFile() return value; only switch
  to canonical path if migration succeeded OR the canonical file already exists
- opencode-config-dir.ts: use CONFIG_BASENAME for omoConfig path in
  getOpenCodeConfigPaths()
- config-context.ts: getOmoConfigPath() now uses detectPluginConfigFile() to
  find whichever name variant actually exists on disk
- plugin-config.ts: default fallback paths use CONFIG_BASENAME instead of
  hardcoded legacy name
- Added test: loadPluginConfig still loads config when migration fails
  (read-only directory simulation)

src/shared/opencode-config-dir.ts

@@ -2,6 +2,8 @@ import { existsSync, realpathSync } from "node:fs"
 import { homedir } from "node:os"
 import { join, resolve, win32 } from "node:path"
 
+import { CONFIG_BASENAME } from "./plugin-identity"
+
 import type {
   OpenCodeBinaryType,
   OpenCodeConfigDirOptions,
@@ -97,7 +99,7 @@ export function getOpenCodeConfigPaths(options: OpenCodeConfigDirOptions): OpenC
     configJson: join(configDir, "opencode.json"),
     configJsonc: join(configDir, "opencode.jsonc"),
     packageJson: join(configDir, "package.json"),
-    omoConfig: join(configDir, "oh-my-opencode.json"),
+    omoConfig: join(configDir, `${CONFIG_BASENAME}.json`),
   }
 }