feat(rag): agent document tools — list/read tools with env-tuned budgets, SSE tool events + "calling tool" UI

Grounded chat turns now run the agent loop (app/rag/agent.py) instead
of a bare chat_stream: while the per-turn budgets last
(BOR_AGENT_LIST_CALLS / BOR_AGENT_READ_CALLS, default 1 each) the model
gets list_documents (the indexed catalog, /api/docs order) and
read_document (full text, never truncated — A7-revised contract); once
both budgets are spent the tools key is dropped from the request and
the model must answer. Rejected calls (unknown tool, unknown/missing
path, document already in context, spent budget) consume no budget.
Budgets 0/0 make exactly one tools=None request — byte-identical to
the pre-phase path (budgets-as-kill-switch). Deflected turns keep the
direct chat_stream (A8 unchanged; the LOW prompt never carries the
<tools> section).

SSE contract gains {"type":"tool","name":...,"argument":
"source/path"|null} frames ahead of the answer deltas (PLAN §4
extension, owner permission 2026-08-26); done.sources, query_log.sources
and the per-turn log line (gains tool_calls=N) report the retrieval
docs + read docs, deduped. The UI shows a "calling tool"
button/label state and one visible .tool-call line per call above the
answer; the lines persist with the chat record and re-render on
reload. chat_stream passes tools through and accumulates streaming
tool_calls deltas into ToolCallPiece (tools=None stays byte-identical).

E2E: deterministic mock tool flow ("use your tools" + <tools> marker:
list -> read first catalog line -> quoted answer) plus the story suite
(marker flow, reload re-render, plain/deflected no-tool regressions).
Docs: .env.example + README (the two tools, the budgets, the SSE tool
frame, the "calling tool" UI state).

probe: turbo tool_calls=supported 2026-08-26 (uv run python -m
scripts.llm_probe --tools — non-streaming + streaming
finish_reason=tool_calls, indexed delta.tool_calls partials)

app/rag/prompts.py

@@ -23,6 +23,12 @@ the ``<tuning>`` section (order: ``<relevance>`` →
 ``<knowledge_base>`` → ``<tuning>`` → mode body) — the agent knows
 roughly what the KB contains before retrieval. With an empty row the
 prompt is byte-identical to the pre-phase text.
+
+Agent tools (phase 37): the **HIGH** prompt only carries a ``<tools>``
+section after the ``<documents>`` body — the grounded turn may call the
+server-side ``list_documents`` / ``read_document`` tools (budgeted, see
+:mod:`app.rag.agent`). The LOW/deflection prompt never carries it and
+stays byte-identical to the pre-phase text.
 """
 from __future__ import annotations
 
@@ -66,6 +72,25 @@ _KB_INTRO = (
     "(generated at import time):\n"
 )
 
+#: The ``<tools>`` instructions section — **HIGH prompt only** (phase 37,
+#: task 03): a grounded turn may extend its context through the two
+#: server-side tools (budgets: ``BOR_AGENT_LIST_CALLS`` /
+#: ``BOR_AGENT_READ_CALLS``, see :mod:`app.rag.agent`). Appended after
+#: the mode body (``<documents>``), so the instructions are the last
+#: thing the model reads. The LOW/deflection prompt never carries it —
+#: a deflection has no grounded context to extend — and stays
+#: byte-identical to the pre-phase text. The E2E mock keys off the
+#: ``<tools>`` marker's *presence*, not this wording.
+TOOLS_SECTION: str = (
+    "<tools>\n"
+    "If the documents in your context reference other files, or you need "
+    "content that is not included above, call `list_documents` to see what "
+    "is indexed, then `read_document` to pull in exactly one more document. "
+    "Answer as soon as you have what you need — do not read more than one "
+    "extra document.\n"
+    "</tools>"
+)
+
 
 def _base(relevance: str) -> str:
     if relevance not in ("HIGH", "LOW"):
@@ -155,11 +180,13 @@ def build_high_prompt(
     kb_overview: str | None = None,
 ) -> str:
     """Grounded turn: locked persona (+ steering, + KB overview) + full
-texts of the top documents.
+texts of the top documents + the ``<tools>`` instructions (phase 37).
 
-    Section order (phase 31): ``<relevance>`` → ``<knowledge_base>`` →
-    ``<tuning>`` → ``<documents>``; empty steering/overview omit their
-    section, keeping the prompt byte-identical to the pre-phase text.
+    Section order: ``<relevance>`` → ``<knowledge_base>`` → ``<tuning>``
+    → ``<documents>`` → ``<tools>``; empty steering/overview omit their
+    section. ``<tools>`` is always present in the HIGH prompt (the
+    budgets — not the prompt — decide whether the tools are actually
+    offered to the model, see :mod:`app.rag.agent`).
     """
     blocks = [
         f'<document source="{doc.source}" path="{doc.path}" title="{doc.title}">\n'
@@ -174,7 +201,7 @@ texts of the top documents.
     for part in (build_kb_section(kb_overview or ""), build_steering_section(notes or [])):
         if part:
             prompt += "\n" + part
-    return prompt + "\n<documents>\n" + body + "\n</documents>"
+    return prompt + "\n<documents>\n" + body + "\n</documents>\n" + TOOLS_SECTION
 
 
 def build_deflect_prompt(