feat: add sandbox filesystem and terminal support

Add sandbox filesystem methods (list_dir, mkdir, remove, upload,
download, stream_upload, stream_download) and interactive PTY sessions
(PtySession, AsyncPtySession) with reconnect support per
FILE_TERMINAL.md spec. Refactor error handling into exceptions.py as
shared handle_response(). Replace API-key-only proxy auth with unified
_proxy_headers() supporting both API key and JWT. Fix stream_upload to
build multipart manually instead of relying on httpx files= with
generators. Switch Makefile SPEC_URL from main to dev branch. Regenerate
models from updated OpenAPI spec (adds teams, channels, metrics, PTY
endpoints). Add comprehensive unit and integration tests. Trim AGENTS.md
to verified facts only.

src/wrenn/exceptions.py

@@ -1,5 +1,7 @@
 from __future__ import annotations
 
+import httpx
+
 
 class WrennError(Exception):
     """Base exception for all Wrenn SDK errors."""
@@ -51,3 +53,51 @@ class WrennAgentError(WrennError):
 
 class WrennInternalError(WrennError):
     """500 — Unexpected server error."""
+
+
+_ERROR_MAP: dict[str, type[WrennError]] = {
+    "invalid_request": WrennValidationError,
+    "unauthorized": WrennAuthenticationError,
+    "forbidden": WrennForbiddenError,
+    "not_found": WrennNotFoundError,
+    "invalid_state": WrennConflictError,
+    "conflict": WrennConflictError,
+    "host_has_sandboxes": WrennHostHasSandboxesError,
+    "host_unavailable": WrennHostUnavailableError,
+    "agent_error": WrennAgentError,
+    "internal_error": WrennInternalError,
+}
+
+
+def handle_response(resp: httpx.Response) -> dict | list:
+    if resp.status_code >= 400:
+        try:
+            body = resp.json()
+        except Exception:
+            resp.raise_for_status()
+            raise
+
+        err = body.get("error", {})
+        code = err.get("code", "internal_error")
+        message = err.get("message", resp.text)
+
+        exc_cls = _ERROR_MAP.get(code, WrennError)
+
+        if exc_cls is WrennHostHasSandboxesError:
+            raise WrennHostHasSandboxesError(
+                code=code,
+                message=message,
+                status_code=resp.status_code,
+                sandbox_ids=body.get("sandbox_ids", []),
+            )
+
+        raise exc_cls(
+            code=code,
+            message=message,
+            status_code=resp.status_code,
+        )
+
+    if resp.status_code == 204:
+        return {}
+
+    return resp.json()