feat(github): seed org membership via orgs[].members

[yonatangross]

Summary

• Add an optional members array to github.orgs[] seed entries: { login, role?: "admin" | "member" }
• Membership is stored as the synthetic members team, the same representation PUT /orgs/:org/memberships/:username writes at runtime, so admin = maintainer of that team and every existing org-role check applies unchanged
• Unknown member logins throw at seed time (they must be seeded users), matching the strict seed validation added for apps in 0.10.0

Why

A seeded org has no members and no admin. Every private repo it owns is therefore unreachable by every seeded user token (403), and nothing at runtime can repair that: the membership endpoint itself requires an org admin. Seeding one admin closes the loop. Hit this in Yonatan-HQ/platform when moving the seed from a user owner to the real org owner.

Changes

File	Change
index.ts	members? on the orgs[] seed type; seedOrgMembers() (team-backed, idempotent per login)
__tests__/org-members.test.ts	roles listed, members read a private org repo / outsiders 403, seeded admin can grant membership at runtime, unknown login throws
README.md, skills/github/SKILL.md	document the field and the "seed one admin" rule

Test plan

• pnpm --filter @emulators/github test (5 files pass, incl. the 4 new cases)
• pnpm -r build
• pnpm --filter @emulators/github lint (0 errors, no new warnings)

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