Adds an `Indexer` subcommand to gnd that forwards commands to the
`graph-indexer` binary from the `indexer-cli` npm package. This lets
users run indexer management commands (allocations, rules, cost models,
etc.) directly through gnd.

- `version`, `help` and their short forms are forwarded as top-level
  commands; all other subcommands get the `indexer` namespace prepended
  (required by graph-indexer's Gluegun plugin system)
- `--help` is handled by clap (works without graph-indexer installed)
- On Unix, uses exec() for clean signal forwarding and exit codes
- Clear error message when graph-indexer is not on $PATH