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/fluksio/sdk/cli.py

@@ -17,9 +17,10 @@ from typing import Any
 
 from fluksio.sdk import FLOWS, Flow, SyncError
 from fluksio.sdk.client import (
+    GLOBAL_DATA_DIR,
     ApiError,
     Client,
-    config_path,
+    data_dir,
     login,
     origin_of,
     repo_root,
@@ -104,13 +105,15 @@ def discover(targets: list[str]) -> list[Flow]:
 
 
 def cmd_login(args: argparse.Namespace) -> int:
+    """Only needed for an engine somewhere else — `serve` signs you in here."""
     email = args.email or input("Email: ")
     password = args.password or getpass.getpass("Password: ")
+    directory = GLOBAL_DATA_DIR.expanduser() if args.shared else data_dir()
     try:
-        login(args.url, email, password)
+        path = login(args.url, email, password, directory=directory)
     except ApiError as exc:
         return _fail(f"could not log in: {exc.detail}")
-    _say(f"Logged in to {args.url}; the token is in {config_path()}.")
+    _say(f"Logged in to {args.url}; the token is in {path}.")
     return 0
 
 
@@ -263,10 +266,18 @@ def add_parsers(subparsers: Any) -> None:
         )
         sub.add_argument("--token", default="", help="override the stored token")
 
-    parser = subparsers.add_parser("login", help="store a token for an engine")
+    parser = subparsers.add_parser(
+        "login", help="store a token for an engine elsewhere"
+    )
     parser.add_argument("--url", default="http://localhost:8000")
     parser.add_argument("--email", default="")
     parser.add_argument("--password", default="")
+    parser.add_argument(
+        "--global",
+        dest="shared",
+        action="store_true",
+        help="store it for the machine rather than this project",
+    )
     parser.set_defaults(func=cmd_login)
 
     parser = subparsers.add_parser(