docs(arch): document context budget fix (#44) and max_turns rationale

Architecture.md

@@ -117,7 +117,25 @@ Cache is reused across runs for the same target. `--fresh` ignores it.
 
 `claude-sonnet-4-20250514`
 
-Context budget: 70% of 180,000 tokens (126,000). Early exit flushes partial
-cache on budget breach.
+**Context budget.** 70% of 200,000 tokens (140,000) — Sonnet 4's real
+context window with a 30% safety margin. The budget is checked against
+the *latest* per-call `input_tokens` reading (the actual size of the
+context window in use), not the cumulative sum across turns. Early
+exit flushes partial cache on budget breach. See #44.
+
+**Per-loop turn cap.** Each dir loop runs for at most `max_turns = 14`
+turns. This is a sanity bound separate from the context budget — even
+on small targets the agent should produce a `submit_report` long
+before exhausting 14 turns. The cap exists to prevent runaway loops
+when the agent gets stuck (e.g. repeatedly retrying a failing tool
+call). If we observe legitimate investigations consistently hitting
+14, raise the cap; do not raise it speculatively.
+
+**Per-loop message history growth.** Tool results are appended to the
+message history and never evicted, so per-turn `input_tokens` grows
+roughly linearly across a loop (~1.5–2k per turn observed on
+codebase targets). At the current `max_turns=14` cap this stays well
+under 200k. Raising `max_turns` significantly (e.g. via Phase 3
+dynamic turn allocation) would expose this — see #51.
 
 Pricing tracked and reported at end of each run.