#3441 swamp start / swamp workspace create: let me choose where a new workspace is created

Problem New workspaces from swamp start (a new name via --workspace, or "create" in the browser page) and from swamp workspace create <name> are always placed under <data dir>/workspaces/<name> (~/.swamp/workspaces by default). There is no per-invocation way to say where the folder should go, for example next to my other projects in ~/dev/.

Current workarounds

• Set SWAMP_HOME / the data dir for the whole process. This moves every workspace and the registry, not one workspace.
• Create the repo myself (swamp init <path>), then swamp workspace import <path>, then swamp start --workspace <name>. This works, but it skips the "create" step that swamp start is built around.

Proposed solution

• swamp workspace create <name> --path <dir> registers and creates the workspace at <dir> (or at <dir>/<name>).
• swamp start --workspace <new-name> --path <dir> does the same when it creates a new name.
• The browser page's "create workspace" form gets an optional location field, defaulting to the data dir.
• Optionally, a persisted setting for the default parent directory (e.g. a workspacesDir config key), so everyday use needs no flag.

The folder would be registered as it is today, so list, get, remove and start status keep working unchanged.

Alternatives considered

• SWAMP_HOME: too coarse, as above.
• Always init + import: works, but is two extra steps and bypasses the start flow.

Environment: swamp 20261014.121640.0-sha.c5d1db96, macOS (Darwin 25.6.0)