docs: consumer docs (README + guides) and one installable prisma-composer skill

[wmadden-electric]

Two audiences, one surface: human guides (canonical) and an installable agent skill (the condensed mirror), plus a consumer-oriented root README linking both.

Human docs

• README.md reoriented toward people building apps on Composer: what a service and root module look like, the real examples/store topology in place of the aspirational diagram, a guides table, an examples table, the agent-skill install. Design docs demoted to a "Design & internals" section. Shipped surface only — unshipped design (stream connections, shared-Postgres contract slices, topology artifact queries) is no longer presented as current.
• docs/guides/ — four guides, canonical for humans:
  • getting-started.md — empty directory to a deployed two-service app
  • building-an-app.md — contracts, databases (postgres / pnPostgres + migrations), authoring reusable Modules, the shared cron/storage/streams modules, config params, secrets, builds
  • testing.md — mockService, bootstrapService, the /fake entry-point pattern
  • deploying.md — stages, destroy semantics, CI, production behavior (gotchas-derived)

Agent skill

One self-contained skill, skills/prisma-composer/, replaces the deploying-prisma-apps + testing-prisma-apps pair and adds the missing authoring story, so npx skills add prisma/composer --skill prisma-composer gives an agent the whole write-test-deploy workflow in one install. skills/README.md records the install command, the authoring rules, and that the guides are canonical (a surface change lands in both).

Shaping decisions (operator-settled): one skill, not a prisma-next-style ten-skill cluster — Composer's surface is small enough; guides/skill duplication is accepted because the audiences differ; describe only what ships.

Fixes riding along:

• skill folder names now match frontmatter names (the runtimes key on frontmatter; the old folders said prisma-apps, the frontmatter said prisma-composers — stale vocabulary)
• no skill link resolves outside the skill folder (the old testing skill linked ../../docs/design/..., which dangles once installed into another repo)
• standaloneEntryPath corrected to standaloneServerPath — the export that actually ships

Verification

• Every @prisma/* import and symbol in the skill and guides checked against the packages/9-public/* export maps and source barrels while drafting; CLI flags checked against packages/0-framework/3-tooling/cli/src/main.ts.
• Trial install end-to-end: npx skills add file://<repo> --skill prisma-composer -a claude-code -y into a scratch project copies the skill to .claude/skills/prisma-composer/.
• Every relative link in README + guides script-checked against the tree; ADR links use the real filenames.
• lint:framework-vocabulary clean.

🤖 Generated with Claude Code

[pkg-pr-new]

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commit: 9d3c7b5