Community trust: expand ADRs and public ecosystem-stabilization positioning

[tonythethompson]

Problem

Nushell's ecosystem is centralized around the core team; technical rigor alone doesn't win adoption without visible, honest positioning and documented decision-making. One ADR exists (0001-ecosystem-trust-upstream-contribution-fork-stewardship.md); the project needs a couple more to cover open decisions as they land, plus README/positioning copy that frames Numan correctly.

Goal

Keep architecture decisions publicly documented as they're made, and make the README's framing explicit: Numan is a stabilization layer over cargo install / nu-scripts fragmentation, not a competitor to Nushell's built-in features.

Scope

• New ADR(s) as Phases 1–3 (CI pipeline: cross-platform pre-built, signed plugin binaries in the official registry #30, One-touch numan migrate command for nupm adopters #31, Invisible activation: single generated numan_init.nu bootstrap file #32, Publish a curated default registry and bootstrap trust policy #18, Nupm migration planner with structured compatibility outcomes #19) land — at minimum one covering the official-registry trust/signing model once CI pipeline: cross-platform pre-built, signed plugin binaries in the official registry #30/Publish a curated default registry and bootstrap trust policy #18 ship, and one covering the numan migrate non-destructive-import guarantee once One-touch numan migrate command for nupm adopters #31 ships.
• README positioning pass: an explicit "why Numan, not just cargo install / nu-scripts" section, once the registry (Publish a curated default registry and bootstrap trust policy #18/CI pipeline: cross-platform pre-built, signed plugin binaries in the official registry #30) and migration (One-touch numan migrate command for nupm adopters #31) stories are real enough to back the claim with working commands, not aspiration.
• Cross-link ADRs from README and docs/PACKAGING.md/Phase7Plan.md so decisions are discoverable, not buried.

Acceptance criteria

• Every non-obvious architectural or trust decision shipped alongside Publish a curated default registry and bootstrap trust policy #18/Nupm migration planner with structured compatibility outcomes #19/CI pipeline: cross-platform pre-built, signed plugin binaries in the official registry #30/One-touch numan migrate command for nupm adopters #31/Invisible activation: single generated numan_init.nu bootstrap file #32 has a corresponding ADR.
• README states Numan's positioning in one paragraph a new user can read before installing anything.

Non-goals

• Marketing copy beyond README/docs — no external blog posts, launch campaigns, etc. tracked here.