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/0008-agent-audit-single-file-invocation.md

@@ -14,3 +14,9 @@
 - Scope detection walks up from the input file: first directory containing `plugin.json` → plugin scope; first directory containing `.git` without `plugin.json` → project scope; path under `~` with neither → user scope.
 - At user scope the derivation crosses filesystem locations (`~/.claude/agents/` ↔ `~/.copilot/agents/`); the script must handle the home directory case explicitly.
 - The invocation signature is the public contract. Changing it is a breaking change to any caller — treat it as such.
+
+**Update (ADR-0016):** the plugin-scope clause above is superseded. Plugin scope is no longer
+detected via `plugin.json`, and there is no counterpart to derive — a directory containing
+`apm.yml` produces a single `.apm/agents/<name>.agent.md` file, and `agent-audit` validates it
+directly with no pair-consistency check. Project scope and user scope keep the pair-derivation
+mechanism described above unchanged. See ADR-0016.