Consolidate session context handling and add global config support

Refactored context file discovery by removing separate SessionContext module and integrating functionality into SystemPrompt.context(). Added support for finding AGENTS.md and CLAUDE.md files in global config directories.

🤖 Generated with [opencode](https://opencode.ai)

Co-Authored-By: opencode <noreply@opencode.ai>

packages/opencode/src/session/system.ts

@@ -1,6 +1,9 @@
 import { App } from "../app/app"
 import { Ripgrep } from "../external/ripgrep"
+import { Global } from "../global"
 import { Filesystem } from "../util/filesystem"
+import path from "path"
+import os from "os"
 
 import PROMPT_ANTHROPIC from "./prompt/anthropic.txt"
 import PROMPT_ANTHROPIC_SPOOF from "./prompt/anthropic_spoof.txt"
@@ -101,7 +104,17 @@ export namespace SystemPrompt {
       const matches = await Filesystem.findUp(item, cwd, root)
       found.push(...matches.map((x) => Bun.file(x).text()))
     }
-    return Promise.all(found)
+    found.push(
+      Bun.file(path.join(Global.Path.config, "AGENTS.md"))
+        .text()
+        .catch(() => ""),
+    )
+    found.push(
+      Bun.file(path.join(os.homedir(), ".claude", "CLAUDE.md"))
+        .text()
+        .catch(() => ""),
+    )
+    return Promise.all(found).then(Boolean)
   }
 
   export function summarize(providerID: string) {