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

docs/reference/configuration.md

@@ -14,7 +14,7 @@ Anything already exported wins over the file.
 
 | Variable | Default | Notes |
 |---|---|---|
-| `DATA_DIR` | `flow-data` (`~/.fluksio` via the CLI) | everything below derives from this |
+| `DATA_DIR` | `flow-data` (`./.fluksio` via the CLI; `~/.fluksio` with `--global`) | everything below derives from this |
 | `DATABASE_URL` | SQLite in `DATA_DIR` | any SQLAlchemy URL |
 | `FLOWS_DIR` | `$DATA_DIR/flows` | the git repository holding flows |
 | `SECRETS_FILE` | `$DATA_DIR/secrets.enc` | encrypted credentials, deliberately outside the repo |