fix(engine): send tool_choice.disable_parallel_tool_use on agentic loop (STOPGAP)

The agentic loop keeps only the FIRST tool_use block per round (chat.el:2281,
"Capture first tool_use block only"). Anthropic lets a model emit several
tool_use blocks in one message and requires a tool_result for every one, so a
parallel-tool turn is answered once, the rest are dropped, and the next request
dies with:

  tool_use ids were found without tool_result blocks immediately after

(neuron#78 quotes this as "tool_use ids found without tool_result"; the above is
the API's actual wording - recorded so the next person's grep matches.)

This constrains the wire to match what the loop can assemble:
  "tool_choice":{"type":"auto","disable_parallel_tool_use":true}

STOPGAP - AND THE DURABLE FIX ALREADY EXISTS. A correct multi-tool loop is
already in Will's EL runtime, in C, and the soul does not call it. Verified on
el:origin/main lang/el-compiler/runtime/el_runtime.c: llm_register_tool:9616,
llm_build_tool_results:9743 - which walks EVERY content block, emits one
tool_result per tool_use, and sets is_error for an unregistered tool -
llm_call_agentic:9817 calling it at :9918, iteration cap 10 at :9847. Will's
commit 12d5e77 (2026-04-30). grep for llm_call_agentic/llm_register_tool across
every neuron/*.el returns nothing; dist/soul.c has zero references. chat.el
hand-rolls its own single-tool loop instead, and that is the one that breaks.

The durable fix is therefore to register the soul's tools via llm_register_tool
and call llm_call_agentic - deleting a loop, not writing one. See ADR 0005.

Our own approved spec called this seven weeks ago:
docs/research/agentic-tool-approval-design.md (2026-06-12, "Approved for build"),
line 20 on the defect, line 30 on the goal ("Execute all tool_use blocks in a
turn (one result per block)").

Two edits, because dist/soul.c cannot be regenerated here (Will's gated elc/elb
toolchain is not on this machine):
  (a) chat.el:2255 - source of truth, so a later regen carries the fix.
      One edit covers all three routes: agentic_loop is called from chat.el:2152
      (/api/chat agentic), :2676 (dharma room) and :2496 (agentic_resume).
  (b) dist/soul.c:28173 - generated form, hand-spliced. Line 27624 is the
      non-agentic/OpenAI-compat req_body (no tools) and was left untouched.
Prior art reused rather than reinvented: soul-narrated-runs-20260713.patch
(27,824 bytes) line 78 spliced this same string into the same concat chain on
2026-07-13.

Deliberately NOT ported from that patch, having read it: max_tokens is not
changed by it (16384 sits on both sides of the hunk; our main's 4096 is a
separate output-truncation concern), and its pause_turn pairing fix - same
defect class - is unreachable today because no server-side web_search is wired
(agentic_tools_with_web at chat.el:1418 is never called), so it is untestable
and logged instead.

Proven E2E on a scratch profile and port 7791, never the live chain. A/B against
a pristine origin/main control built from the same vendored runtime: fixed
completed the mission (tools_used read_file x3, 4 iterations, correct answer);
control failed 3/3. Direct API probe confirmed the mechanism - without the field
the model emits 3 parallel tool_use blocks and replaying the unfixed loop's next
turn returns HTTP 400; with it, exactly 1 block.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

chat.el

@@ -2243,8 +2243,16 @@ fn agentic_loop(session_id: String, model: String, safe_sys: String, tools_json:
     }
 
     while keep_going && iteration < 8 {
+        // STOPGAP (2026-08-03, neuron#78 bug b / ADR 0005): the block walk below keeps
+        // only the FIRST tool_use block per round, so a PARALLEL tool_use response leaves
+        // the other ids without tool_result and the next turn 400s with
+        // "tool_use ids found without tool_result" — killing the run. Sending Anthropic's
+        // documented tool_choice.disable_parallel_tool_use makes the wire contract match
+        // what this loop can actually assemble. The real fix — a multi-tool_result loop
+        // that answers every block in a round — is Will's; see neuron#78.
         let req_body: String = "{\"model\":\"" + model + "\""
             + ",\"max_tokens\":4096"
+            + ",\"tool_choice\":{\"type\":\"auto\",\"disable_parallel_tool_use\":true}"
             + ",\"system\":\"" + safe_sys + "\""
             + ",\"tools\":" + tools_json
             + ",\"messages\":" + messages