feat(rag): unbounded agent tool calls behind a round cap (owner revision)

Phase 45 (owner permission 2026-08-27, TODO.md L8: "allow the LLM
to make as many tool calls as it wants"): the phase-37 per-turn tool
budgets (BOR_AGENT_LIST_CALLS / BOR_AGENT_READ_CALLS, default 1 each)
and their exhaustion refusals are removed — a grounded turn now offers
list_documents / read_document for the whole turn (re-lists included),
bounded only by the round cap:

- app/config.py: agent_max_rounds (BOR_AGENT_MAX_ROUNDS, default 10,
  negative rejected) replaces agent_list_calls / agent_read_calls;
  .env.example + README document the single knob; app/rag/prompts.py
  docstrings follow.
- app/rag/agent.py: the loop runs tools until the model answers or
  rounds >= max_rounds, at which point it forces one final no-tools
  answer (the cap is the only forced exit); 0 = no tools — exactly one
  tools=None request, byte-identical to the pre-phase-37 path (the
  kill switch). Rejected calls (unknown tool / missing args /
  already-in-context / unknown path) still consume a round, so
  pathological rejected-call streams are bounded by the cap. The
  per-call log line is now tool/args/round=N/M; the per-turn
  tool_calls=N field and the tool SSE event are unchanged.
- tests/e2e/mock_llm.py: MULTI_READ_TRIGGER ("read two documents") —
  the deterministic list -> read #1 -> read #2 -> forced-answer flow
  (byte-stable "I read <sp1> and <sp2>." line), classified by the
  count of tool-role read results; the phase-37 single-read flow stays
  byte-identical (unit-pinned in tests/unit/test_mock_tool_flow.py).
- tests/e2e/test_agent_unlimited_tools.py (new, story suite,
  mock-only): three tool frames/lines in order (one list, two reads —
  the second read is what the old read budget refused) + the
  both-named non-deflected answer; done.sources + chips = retrieval
  doc + both reads, deduped; no budget refusal rendered; the
  single-read marker flow regression (exactly one read, single tool
  pair).
- .agent/PLAN.md: the phase-45 SSE revision note (owner-locked, R2) —
  the only PLAN edit this phase; the phase-37 note's budget clause is
  marked removed.

Unit/integration rewrites (test_agent.py round-cap matrix incl. the
kill switch and rejected-call spam, test_config.py, test_chat_api.py
agent_max_rounds=0 fixtures) landed with the server core so every gate
stays green.

uv run pytest: 756 passed, app/ coverage 99%; ruff + pyright clean;
story E2E 4/4 in isolation (ran twice); regression E2E suites
(agent_document_tools unmodified, chat_rag, smoke) green in isolation.

Also records the 45_agent_unlimited_tools todo/ -> complete/ task-file
moves (00/01/02 pending in the working tree, task 03 moves on success).

.env.example

@@ -38,8 +38,7 @@ BOR_HYBRID_LEXICAL_CANDIDATES=30  # FTS list width for the fusion
 BOR_RRF_K=60                      # Reciprocal Rank Fusion damping constant
 
 # --- Agent document tools (phase 37: grounded turns may list + read) ---
-BOR_AGENT_LIST_CALLS=1            # per-turn list_documents opportunities (0 disables the tool)
-BOR_AGENT_READ_CALLS=1            # per-turn read_document opportunities (0 disables the tool)
+# BOR_AGENT_MAX_ROUNDS=10        # hard cap on agent tool rounds per turn (0 = no tools)
 
 # --- Import scope (A9 formats; may only narrow, never widen) ---
 # BOR_IMPORT_EXTENSIONS=md,markdown,txt,yaml,yml,json,py