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/cli/config-manager/config-context.ts

@@ -1,4 +1,4 @@
-import { getOpenCodeConfigPaths } from "../../shared"
+import { getOpenCodeConfigPaths, detectPluginConfigFile } from "../../shared"
 import type {
   OpenCodeBinaryType,
   OpenCodeConfigPaths,
@@ -42,5 +42,8 @@ export function getConfigJsonc(): string {
 }
 
 export function getOmoConfigPath(): string {
+  const configDir = getConfigContext().paths.configDir
+  const detected = detectPluginConfigFile(configDir)
+  if (detected.format !== "none") return detected.path
   return getConfigContext().paths.omoConfig
 }