docs: the governing principle — every LLM request is reconstructable from the session log

The reconstructability RFC is the principle's home: model-visible ⟺
logged in both forms, the mechanism (boundary derivation + header
fold), the enforcement (write-time round-trip guard, the dev
invariant), the corollaries ranked (prefix-cache stability first), the
MiniCode lineage with the provenance arrow inverted, and the
alternatives it beat — including the stateful transmission client
whose three-design archaeology lives in PR #162.

Placements per the one-home-per-fact taxonomy: a standing-order line in
root AGENTS.md (with displacement trims to stay inside the 1,575-word
ceiling), the principle statement in architecture.md § Session Log and
its Turn Flow lines (condensed to the ratcheted 1,630 ceiling), the
request-envelope section in core-data-structures/core.md with the
LlmCallConfig paste, both review-requested FIXMEs
(FIXME(call-config-shape) beside the type, FIXME(catalog-verbs) at the
catalog's drift-gate note), cookbook rows redirected off agent/request
(tool filtering → system-prompt/assemble, plan-mode prompt → sections/
inject()), and the llm/stream JSDoc stating the frozen-request
contract. RFC index and all generated catalogs regenerated.

docs/architecture.md

@@ -71,8 +71,8 @@ forever:
       assemble system prompt and tool schemas
       agent/pre-step
       'step/start'
-      derive messages from the session log
-      agent/request -> llm/stream
+      snapshot the derived messages (the reconstruction boundary)
+      agent/request (config only) -> log request/header -> llm/stream (frozen)
         'assistant/chunk'
       agent/step-result
       'assistant/message'
@@ -108,6 +108,8 @@ Every session event is turn-enclosed. Reloading a crashed session preserves the
 
 The session log is the source of truth. `deriveMessages()` projects session events into the `Message[]` sent to the model; raw `assistant/chunk` events stay in the log for replay and UI fidelity. Replay, fork, resume, transcript rendering, telemetry, and persistence all derive from the same event stream.
 
+**Model-visible ⟺ logged**: the log reconstructs every conversation request byte-for-byte — messages by derivation at the `step/start` boundary, the header (system prompt, tools, model + sampling) by folding `request/header` events — asserted per request by the dev invariant ([reconstructability RFC](rfc/implemented/architecture/2026-07-05-reconstructable-requests.md)).
+
 Durability is a plugin concern. Persistence backends buffer synchronous `session/event` notifications and the loop awaits a turn-end checkpoint before moving on. The `SessionPersistence` seam stores `SessionEvent` directly, with metadata in `SessionHeader`; JSONL and SQLite share one contract suite.
 
 ### Model Content