feat(strategist) S2: graph_overview / source_coverage / marginal_yield / budget_status

DESIGN_STRATEGIST.md §2. Four read-only view tools the strategist uses
to ground its decision each round.

  graph_overview()      — hypotheses table (log_odds, conf, edges_in,
                          distinct_sources, recent_flip), sources table,
                          pending leads. distinct_sources is the
                          critical signal: a hypothesis with 23 edges
                          but only 1 distinct_source has fragile cross-
                          source independence and is a candidate for
                          a corroboration-seeking lead.
  source_coverage(src)  — per-source ✓/✗ against an expected-artefact
                          catalogue. Catalogue is heuristic hints,
                          NOT a forced checklist. Footer reminds the
                          strategist to investigate ✗ items only when
                          an active hypothesis depends on them — this
                          is the "应试能力存在但不被绑死" guardrail.
  marginal_yield(N)     — new phenomena / edges / status flips per
                          recent round. Two consecutive zero-yield
                          rounds = strong signal to declare complete.
  budget_status()       — usage vs caps (tool_calls, rounds, wall
                          clock). Pacing warnings at 70% / 90%.

tools/strategy.py also exports EXPECTED_ARTEFACTS, a per-source-type
table of (name, detector, value_for) entries. Detectors are
substring patterns on tool name + args; the matcher resolves at
call time against graph.tool_invocations. Catalogue covers iOS /
Android / Windows disk / media-collection / archive source types.

All four tools registered in tool_registry, listed as read-only in
llm_client.READ_ONLY_TOOLS for parallel execution. They go through
the invocation-logging wrapper so the strategist's reads are
themselves auditable (the wrapper does NOT cache them — graph
state changes between calls).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

llm_client.py

@@ -148,6 +148,8 @@ READ_ONLY_TOOLS: set[str] = {
     "parse_ios_keychain", "read_idevice_info",
     # Android + media reads (S6) — set_active_partition is NOT read-only.
     "probe_android_partitions", "ocr_image",
+    # Strategist view tools (DESIGN_STRATEGIST.md §2) — pure renders.
+    "graph_overview", "source_coverage", "marginal_yield", "budget_status",
 }