docs(superpowers): add Stage 0 multi-tenant foundation master plan
8 PR 串成的 stage-level 拆解(Postgres 切换 + workspaces schema + auth 扩字段 + 入口路由强校验 + CI boundary + headless schema 预留)。每个 PR 含 commit-sized TDD task list 和验收清单。执行时各 PR 独立 review checkpoint。 Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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