docs(adr): supersede 0004 with 0005 vendor-binaries implementation plan

[johnnyhuy]

Summary

ADR-0004 reserved the decision to bundle bw/op/sops/age but left sourcing, layout, signing, and resolution semantics as follow-ups. ADR-0005 replaces it with the concrete plan.

Changes

• ADR-0004 → superseded. Frontmatter now points at 0005.
• ADR-0005 (new). Implementation-ready plan covering what bundles vs. what ships as an npm dep, the resources/bin/<platform>-<arch>/ layout, the build-time fetch (pinned vendor-versions.json, sha256 verification), runtime resolution, lazy backend loading, macOS re-signing for Gatekeeper, and a diagnostics row per binary.
• Resolution order deliberately reversed from 0004: env override → $PATH → bundled. Reasoning in the ADR: bw and op carry the user's auth state (BW_SESSION, biometric unlock, op service-account token), so the installed copy wins. sops/age are stateless so bundling wins on convenience — but uniform PATH-first is easier to reason about and the env override gives power users an escape hatch.

Consequences

• +~50 MB macOS / +~35 MB Linux / +~30 MB Windows to app size. sops is the largest single contributor.
• vendor-versions.json becomes a chore; bumps ride Hoist releases for v1.
• macOS CI gains a DEVELOPER_ID_APPLICATION + notarisation dependency. Linux/Windows dev builds stay unsigned (no Gatekeeper).
• People who read ADR-0004 in isolation will be surprised by the resolution-order flip. Mitigated by the frontmatter link.

Testing

• This is docs only — no CI run.

Refs: ADR-0001 (backend abstraction), ADR-0004 (superseded stub).