Add shell completion for blob store IDs

[friedenberg]

Summary

3 TODOs request completion support for blob store IDs in commands that accept
them. Currently these commands have no tab-completion for store IDs.

Affected files

File	Command
internal/india/commands_madder/sync.go:54	madder sync
internal/india/commands_madder/fsck.go:28	madder fsck
internal/victor/commands_dodder/repo_fsck.go:30	dodder repo-fsck

Notes

• Blob store IDs have a known format: unprefixed (XDG user), . prefix (CWD),
  / prefix (XDG system)
• The completion should enumerate available store IDs from the current repo's
  config and XDG directories
• Look at existing completion patterns in commands_dodder/complete.go and
  commands_madder/complete.go for how to wire this up

[friedenberg]

Implemented in 9b8f851e3. Added Complete methods to sync, fsck, and repo-fsck commands. BATS tests added for all three.