Rename the import package app to fluksio

A wheel whose top-level module is `app` collides with anything else in a
user's venv, so the package that is about to be published takes the name
it is published under. Only the Python package moves; the repo, the
Docker WORKDIR and the compose project keep theirs.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

backend/fluksio/flow/nodes/file.py

@@ -0,0 +1,98 @@
+"""File: read a file into the graph, or write one out of it.
+
+Confined to a directory the engine owns. A flow that could name any path
+would be a way to read the secrets store or overwrite a node's source.
+"""
+
+from __future__ import annotations
+
+import json
+import logging
+from collections.abc import Iterable
+from pathlib import Path
+from typing import Any, Literal
+
+from pydantic import BaseModel, ConfigDict, Field
+
+from fluksio.flow.messages import MessageSpec
+from fluksio.flow.nodes.base import Node
+
+logger = logging.getLogger(__name__)
+
+
+def files_root() -> Path:
+    """Where flow-readable files live: beside the flow store, not inside it."""
+    from fluksio.core.config import settings
+
+    return settings.FLOWS_DIR.parent / "files"
+
+
+def resolve(name: str) -> Path:
+    """Turn a flow-supplied name into a path inside the sandbox, or refuse."""
+    root = files_root().resolve()
+    root.mkdir(parents=True, exist_ok=True)
+    target = (root / name).resolve()
+    if target != root and root not in target.parents:
+        raise ValueError(f"'{name}' is outside the files directory")
+    return target
+
+
+class FileNode(Node):
+    """Read or write a file under the engine's files directory."""
+
+    class Params(BaseModel):
+        model_config = ConfigDict(extra="allow")
+
+        path: str = Field(description="Path relative to the engine's files directory.")
+        mode: Literal["read", "write", "append"] = "read"
+        format: Literal["text", "json"] = Field(
+            default="text", description="Parse or serialize the contents as JSON."
+        )
+        newline: bool = Field(
+            default=True, description="End each written record with a newline."
+        )
+
+    __slots__ = ("cfg",)
+
+    def __init__(
+        self,
+        requires: MessageSpec | Iterable[MessageSpec] = (),
+        provides: MessageSpec | Iterable[MessageSpec] = (),
+        params: dict[str, Any] | None = None,
+        name: str | None = None,
+    ):
+        self.cfg = self.Params.model_validate(params or {})
+        super().__init__(
+            f=self._act,
+            requires=requires,
+            provides=provides,
+            params=params,
+            name=name or f"file_{self.cfg.mode}",
+        )
+
+    @property
+    def idempotent(self) -> bool:  # type: ignore[override]
+        # Reading twice is harmless; appending twice writes the line twice.
+        return self.cfg.mode == "read"
+
+    def _act(self, params: dict[str, Any], **kwargs: Any) -> dict[str, Any] | None:
+        target = resolve(self.cfg.path)
+
+        if self.cfg.mode == "read":
+            if not target.exists():
+                raise FileNotFoundError(f"'{self.cfg.path}' does not exist")
+            raw = target.read_text()
+            value = json.loads(raw) if self.cfg.format == "json" else raw
+            return {spec.port: value for spec in self.output_ports} or None
+
+        if not kwargs:
+            return None
+        value = next(iter(kwargs.values()))
+        text = json.dumps(value) if self.cfg.format == "json" else str(value)
+        if self.cfg.newline:
+            text += "\n"
+
+        target.parent.mkdir(parents=True, exist_ok=True)
+        with target.open("a" if self.cfg.mode == "append" else "w") as handle:
+            handle.write(text)
+        return None