One installation per project, and no login to reach it

Two things a local install should not have asked for.

`fluksio serve` now signs you in. Logging in to your own machine was a
formality — the password was printed by the same process that would have
checked it, and the database it authenticates against sits in the directory
the token goes into — so `serve` mints the token itself and says where it put
it. `fluksio login` is left for an engine somewhere else.

And an installation is `.fluksio` beside the code, found the way `.git` is,
rather than one `~/.fluksio` for the machine. A repository with its own venv
was already getting its own engine; it now gets its own flows, run history and
token too, instead of three repositories sharing one database and fighting
over one port. `--global` asks for the shared one, `--data-dir` still names
any directory, and when both exist the banner says which you are looking at
and how to reach the other.

The directory ignores itself from within — a `.gitignore` of `*`, the way uv
writes one into `.venv` — because it holds a credential and a database, and
neither belongs in anybody's history. The token is written mode 600. A login
an older version wrote to ~/.config/fluksio is still read, so nothing that
worked stops working.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012ue1tkFWB1bcGy3aWhCKpU

backend/tests/sdk/test_paths.py

@@ -0,0 +1,91 @@
+"""Which installation a command is for, and where its token lives.
+
+A repository with its own venv gets its own engine, so "which one" is a fact
+about the working directory rather than about the machine.
+"""
+
+import json
+from pathlib import Path
+
+import pytest
+
+from fluksio.sdk import client
+
+
+@pytest.fixture
+def elsewhere(tmp_path: Path, monkeypatch: pytest.MonkeyPatch) -> Path:
+    """A working directory with no installation above it."""
+    monkeypatch.chdir(tmp_path)
+    monkeypatch.setenv("XDG_CONFIG_HOME", str(tmp_path / "xdg"))
+    monkeypatch.setattr(client, "GLOBAL_DATA_DIR", tmp_path / "home" / ".fluksio")
+    return tmp_path
+
+
+def test_a_project_local_installation_is_found_from_below(elsewhere: Path):
+    (elsewhere / ".fluksio").mkdir()
+    deep = elsewhere / "src" / "pkg" / "sub"
+    deep.mkdir(parents=True)
+
+    assert client.find_data_dir(deep) == elsewhere / ".fluksio"
+    assert client.config_path(client.data_dir(deep)).parent.name == ".fluksio"
+
+
+def test_the_nearest_one_wins(elsewhere: Path):
+    """An installation inside another belongs to the directory it is in."""
+    (elsewhere / ".fluksio").mkdir()
+    inner = elsewhere / "inner"
+    (inner / ".fluksio").mkdir(parents=True)
+
+    assert client.find_data_dir(inner) == inner / ".fluksio"
+
+
+def test_with_none_above_it_the_shared_one_answers(elsewhere: Path):
+    assert client.find_data_dir() is None
+    assert client.data_dir() == elsewhere / "home" / ".fluksio"
+
+
+def test_a_token_is_written_only_for_its_owner(elsewhere: Path):
+    path = client.write_config("http://127.0.0.1:8000", "abc", elsewhere / ".fluksio")
+
+    assert json.loads(path.read_text()) == {
+        "url": "http://127.0.0.1:8000",
+        "token": "abc",
+    }
+    # It is a credential sitting in somebody's project directory.
+    assert path.stat().st_mode & 0o077 == 0
+    # And one nothing can commit by accident.
+    assert (path.parent / ".gitignore").read_text().endswith("*\n")
+
+
+def test_the_credential_beside_the_data_is_the_one_used(elsewhere: Path):
+    client.write_config("http://127.0.0.1:8131", "local", elsewhere / ".fluksio")
+
+    assert client._stored()["token"] == "local"
+
+
+def test_a_login_from_before_installations_were_local_still_works(elsewhere: Path):
+    """`fluksio login` wrote to XDG once; that must not stop answering."""
+    legacy = client._legacy_config_path()
+    legacy.parent.mkdir(parents=True)
+    legacy.write_text(json.dumps({"url": "http://elsewhere:8000", "token": "old"}))
+
+    assert client._stored()["token"] == "old"
+
+
+def test_a_local_credential_wins_over_the_legacy_one(elsewhere: Path):
+    legacy = client._legacy_config_path()
+    legacy.parent.mkdir(parents=True)
+    legacy.write_text(json.dumps({"url": "http://elsewhere:8000", "token": "old"}))
+    client.write_config("http://127.0.0.1:8131", "local", elsewhere / ".fluksio")
+
+    assert client._stored()["token"] == "local"
+
+
+def test_ignoring_itself_leaves_an_existing_gitignore_alone(elsewhere: Path):
+    directory = elsewhere / ".fluksio"
+    directory.mkdir()
+    (directory / ".gitignore").write_text("mine\n")
+
+    client.ignore_self(directory)
+
+    assert (directory / ".gitignore").read_text() == "mine\n"