Backfill architecture decision records

Seven ADRs capturing the why behind decisions already made: vendoring
Cordis as source with a guarded manifest; the microkernel event
taxonomy with one swappable concrete loop; event-sourced sessions
with derived history and the append-before-emit ordering contract;
the provider-neutral content-block vocabulary (and why not
OpenAI/Anthropic shapes); the custom tool-schema DSL over schemastery;
tool schemas living in the prompt assembly; and mechanical quality
gates over prose guidelines (the agents-write-the-code rationale).

docs/adr/0004-own-content-block-vocabulary.md

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+# ADR 0004: Provider-neutral content-block vocabulary owned by dsh-llm
+
+Status: accepted (2026-06-11)
+
+## Context
+
+The harness needs one internal language for messages that the loop, session
+log, and all plugins speak. Options: mirror the DeepSeek/OpenAI
+chat-completions shape (zero mapping for the first provider, awkward for rich
+content), adopt Anthropic's Messages block structure verbatim (battle-tested,
+but our canonical types would mirror a third-party API we don't target
+first), or own a vocabulary.
+
+## Decision
+
+Own it: messages are arrays of typed content blocks (`text`, `reasoning`,
+`tool-call`, `tool-result`, `image`), with the union derived from the
+merge-extensible `ContentBlockMap` so plugins add block types via declaration
+merging. The same merge-extensible-map pattern types every "stringly" field
+(`MessageSource`, `FinishReason`, `TurnTrigger`, `TurnEndReason`). Streaming
+is a raw chunk protocol; `BlockAssembler` is the single shared assembly
+implementation. Adapters translate to provider wire formats — mapping cost
+lives in adapters, where it belongs.
+
+In-session context injection (`context/message`, `steering/message`) renders
+as tagged user-role envelopes (the system-reminder pattern) rather than a new
+role, so adapters carry zero burden. TODO(review): revisit once the DeepSeek
+V4 adapter exists.
+
+## Consequences
+
+- Reasoning, prefill, cache hints, and multimodal content all have a home
+  without provider contortions.
+- Every adapter pays a translation cost; the streaming protocol carries a
+  TODO(review) marker until the first real adapter validates it.
+- IDs that cross package boundaries are branded (`CallId`, `SessionId`,
+  `AgentId`) — nominal typing at zero runtime cost.