feat: add project groups and --group filter

[temp]

What

Groups are named sets of projects defined in the config file. They can be targeted with --group instead of spelling out directories every time.

projects:
  api: git@github.com:org/api.git
  web: git@github.com:org/web.git
  libs/shared: git@github.com:org/shared.git

groups:
  frontend: [web]
  backend: [api, libs/shared]

commands:
  migrate:
    cmd: npm run migrate
    groups: [backend]

gogo exec --group backend "npm test"
gogo exec --group backend,frontend "npm test"   # union of both groups
gogo git status --group backend                 # available on every filter command
gogo run migrate                                # uses the groups from the command definition

JSON works the same way. Like the other filters, --group is also available as a persistent root flag, and it combines with --include-only / --exclude-only / the patterns via AND. A --group flag on the command line replaces the groups from a command definition.

Implementation

• internal/config: MetaConfig.Groups, CommandConfig.Groups, ResolveGroups (union, deduplicated, sorted), GetGroupNames / GetGroup; merge rule for overlays (the overlay wins per group name); RemoveProject keeps groups consistent.
• internal/filter: new first filter stage Options.GroupOnly — exact path match, no basename match, since group members are config keys.
• internal/cli: --group flag in addFilterFlags and in the root persistent flags; resolveFilterOptionsWithConfig resolves group names against the merged config and only reads the config when --group is used.

Design decision: references are resolved lazily

Group and command cross-references are not validated while reading a config. Otherwise a group referencing a project that lives in an overlay which is not loaded would break every unrelated command — that runs against the whole point of overlays. Instead:

• Validate only checks a single file structurally (name not empty, at least one member).
• ResolveGroups reports bad references when a group is actually used: unknown group "nope". Available groups: backend, frontend.
• gogo validate checks all references against the merged config (config.ValidateReferences), which also surfaces errors that used to be swallowed silently.

Tests

go test -race ./... passes, go vet is clean. New tests in internal/config/config_test.go, internal/filter/filter_test.go, internal/cli/group_test.go and internal/cli/validate_test.go: resolving single and multiple groups, dedupe, unknown group, intersection with include-only / exclude-only, groups from JSON and YAML, overlay merge, write/read round-trip, command groups, RemoveProject cleanup, lazy reference resolution, and the gogo validate report.

Also verified end-to-end with the built binary against a real project, including a group defined in an overlay and a broken reference. make lint did not run locally — golangci-lint is not installed here; CI covers it.

🤖 Generated with Claude Code