Executable coverage for bootstrapped generate targets, and a profile contract for make check

[jeremy]

Two items split out of #5, which reconciled the Makefile target vocabulary across the seed's authoring sources.

1. Nothing verifies that a bootstrapped generate target actually runs

seed/Makefile.tmpl now has <prefix>-generate-services for all five languages, with swift-generate-services an alias for swift-generate (both expand to $(MAKE) -C swift generate). That satisfies MAKEFILE-CONVENTION.md.

What is missing is coverage, and it is not Swift-specific. The seed ships no generator artifact for any language:

Recipe	Artifact it invokes	In the seed?
go-generate-services	go/cmd/generate-services/	absent
rb-generate-services	ruby/scripts/generate-services.rb	absent
kt-generate-services	kotlin/generator/ (gradle)	absent
swift-generate	swift/Makefile generate target	absent

All five are produced by the bootstrap scaffolding step, so on a pristine seed make go-generate-services and make swift-generate fail identically (Error 2). Nothing catches a scaffolding step that silently fails to produce its generator, and nothing catches a conventional target regressing to a no-op.

Wanted: a check that each <prefix>-generate-services target actually runs after bootstrap — asserting the generator artifact exists and the target does work, rather than exiting 0 vacuously. That is what should have gated the Swift alias, and it applies to the other four equally.

Swift's one real asymmetry, worth keeping in mind but not the driver here: it is the only generation target that delegates to a sub-Makefile instead of running the generator inline, so no root-level recipe describes what has to exist.

2. make check has no profile contract

seed/Makefile.tmpl:407 makes check depend on go-check ts-check rb-check swift-check kt-check conformance unconditionally, while prompts/seed-sdk.md offers a single-language profile. A narrower SDK therefore fails the gate on a language it never initialized.

The same applies to the aggregate generate-services, which runs all five generators unconditionally.

Wanted: a profile contract — how the selected language set is expressed, whether the Makefile is trimmed at instantiation or reads a variable, and how conformance behaves on a partial profile. #5 documents the constraint in seed/AGENTS.md.tmpl and seed/CONTRIBUTING.md.tmpl in the meantime.