Add RFCs for the remaining quality-proposal ideas

Eight proposals grouped by category, each with problem statement,
concrete plan, and risks: property-based testing over the
protocol-shaped core (chunk streams, event logs, schema DSL);
mutation testing as the counterweight to the 100%-coverage gate;
deterministic tests + a universal replay-invariant fixture + nightly
race stress; architectural conformance (dependency-cruiser rules and
the LlmAdapter conformance kit); runtime arg validation at the model
boundary with a structured error taxonomy and dev-mode invariants;
doc-sync enforcement (typechecked doc snippets, API reports);
supply-chain checks and nightly vendor-drift verification against the
manifest; and deep-readonly public surfaces (logged-vs-in-flight
mutability boundary). AGENTS.md points at docs/adr and docs/rfc.

AGENTS.md

@@ -29,7 +29,9 @@ packages/    Harness packages, all named @deepseek-ai/dsh-<name>:
   agent-loop/     THE concrete plugin: LoopAgent + the loop driver
 examples/    Runnable demos (not workspaces). echo-agent = mock model + echo
              tool + stdio UI + JSONL persistence, wired via cordis.yml.
-docs/        architecture.md — the design doc.
+docs/        architecture.md — the design doc. adr/ — decision records (the
+             why behind vendoring, event-sourcing, the schema DSL, …).
+             rfc/ — proposals for substantial future work.
 scripts/     build.ts — dumble JS bundling for all packages.
 ```