Use convention-based store paths to eliminate registry file

[krisrowe]

Problem

stores create <name> already creates the bare repo at ~/.dotfiles-<name> by convention, but a registry file is still required to map store names to paths. This adds a config dependency that complicates new-machine setup and is unnecessary when the path follows the convention.

Proposal

Make the registry file optional by adopting convention over configuration:

• Default store: ~/.dotfiles (unchanged)
• Named stores: ~/.dotfiles-<name>
• stores list: Discover stores by scanning for ~/.dotfiles-* directories that are valid bare git repos
• --store <name>: Resolve to ~/.dotfiles-<name> directly, no registry lookup needed
• Registry file: Keep as an optional override for non-standard paths, but don't require it for the common case

Benefit

• One fewer file to bootstrap on a new machine
• stores list works without any config — just discovers what's there
• Simpler mental model: the store name is the directory suffix