feat(shared): add findProjectOpencodePluginConfigFiles walker

Walks up the directory tree from a start directory looking for
.opencode/oh-my-openagent.json[c] (or legacy basename) files. Returns
detected paths in nearest-first order, optionally stopping at a
caller-provided directory.

Reuses detectPluginConfigFile so canonical/legacy basename detection,
caching, and JSONC vs JSON precedence stay consistent with existing
config loading. Foundation for #417 hierarchical config discovery.

src/shared/project-discovery-dirs.ts

@@ -2,6 +2,8 @@ import { execFileSync } from "node:child_process"
 import { existsSync, realpathSync } from "node:fs"
 import { dirname, join, resolve } from "node:path"
 
+import { detectPluginConfigFile } from "./jsonc-parser"
+
 const worktreePathCache = new Map<string, string | undefined>()
 
 function normalizePath(path: string): string {
@@ -114,3 +116,35 @@ export function findProjectOpencodeCommandDirs(startDirectory: string, stopDirec
     stopDirectory ?? detectWorktreePath(startDirectory),
   )
 }
+
+export function findProjectOpencodePluginConfigFiles(
+  startDirectory: string,
+  stopDirectory?: string,
+): string[] {
+  const paths: string[] = []
+  const seen = new Set<string>()
+  let currentDirectory = normalizePath(startDirectory)
+  const resolvedStopDirectory = stopDirectory ? normalizePath(stopDirectory) : undefined
+
+  while (true) {
+    const opencodeDirectory = join(currentDirectory, ".opencode")
+    if (existsSync(opencodeDirectory)) {
+      const detected = detectPluginConfigFile(opencodeDirectory)
+      if (detected.format !== "none" && !seen.has(detected.path)) {
+        seen.add(detected.path)
+        paths.push(detected.path)
+      }
+    }
+
+    if (resolvedStopDirectory === currentDirectory) {
+      return paths
+    }
+
+    const parentDirectory = dirname(currentDirectory)
+    if (parentDirectory === currentDirectory) {
+      return paths
+    }
+
+    currentDirectory = normalizePath(parentDirectory)
+  }
+}