docs(kyberforge): add ADR-0016, agent tools-field omitted at plugin scope

#89's grilling session surfaced a real upstream gap: APM's agent
primitive has no per-target compile integrator, so tools: and every
Claude-only field (isolation, maxTurns, effort, memory, permissionMode)
would verbatim-copy into Copilot's file with an incompatible or
unrecognized shape. Omitting them entirely is the only option that
never ships a wrong value to a real harness.

Scoped to plugin-scope agent-author only — project/user scope keep
today's Claude+Copilot pair model unchanged. Partially supersedes
ADR-0005 and ADR-0008's plugin-scope clauses (addenda appended to
both); ADR-0009's field-inventory.md mechanism is unaffected, only
its plugin-scope content shape changes.

Refs: #89

docs/adr/0005-agent-author-dual-provider-scaffold.md

@@ -44,3 +44,8 @@ separate single-provider skill, adding complexity with no benefit.
 file now lives at `<plugin-root>/sources.md`, outside the `agents/` directory, because
 `claude plugin validate --strict` auto-discovers every `.md` under `agents/` as an agent
 requiring frontmatter. See ADR-0010 for the empirical finding and rationale.
+
+**Update (ADR-0016):** the plugin-scope clause above is superseded. Plugin scope is no longer
+detected via `plugin.json`, and no longer produces a Claude+Copilot file pair — a directory
+containing `apm.yml` now gets a single vendor-neutral `.apm/agents/<name>.agent.md` file with
+no provider-specific fields. Project scope and user scope are unaffected. See ADR-0016.