feat(factory): add native Droid plugin packaging (#120)

* feat(factory): add native Droid plugin packaging

* test(factory): guard documented install contract

* fix(factory): enforce install command order

docs/adr/0008-native-host-manifests-share-one-plugin-root.md

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+# ADR 0008 — Native host manifests share one plugin root
+
+**Status:** accepted (v2.5.14)
+
+## Context
+
+Diagram Design serves Claude Code, Codex, Pi, and Factory Droid. Factory can translate the Claude plugin layout, but relying on that fallback leaves Droid installation undocumented and outside the package-version gate. Copying the skill or commands into host-specific directories would create multiple sources of truth.
+
+## Decision
+
+Claude, Codex, and Factory each receive the smallest native manifest and marketplace metadata their host needs. Every marketplace resolves to the repository root, where all three hosts reuse `skills/diagram-design/` and `commands/` without duplication. Pi continues to use the same root package surfaces.
+
+The three native plugin manifests carry identical shared identity, description, version, author, repository, license, and keyword metadata. The package verifier rejects drift, deletion, unsafe marketplace paths, or a version that does not advance from the base ref. A newly tracked native manifest may be absent at the base ref only during bootstrap: its current metadata and version must match the established manifests, and those established manifests must advance.
+
+## Consequences
+
+Each native host has an explicit install path while diagram behavior remains single-sourced. Adding another host requires native metadata, package-gate coverage, documentation, and a synchronized version bump; it never justifies copying the skill or command surface. Git-based Factory installs are updated by marketplace commit, while the synchronized manifest version remains release metadata and a review gate.