refactor: single ACTIVITIES: block with bullet-level trim (T71.2)

Phase 2 T43 added a SECOND ACTIVITIES: block to render guest activity
separately from you+speaker. Two consecutive ACTIVITIES: headers can
read as a duplicate-section bug to the LLM and bloat the prompt.

Consolidate to a single ACTIVITIES: block whose body is composed from
up to three bullets (you, speaker, guest). The block itself is
MUST-tier (always renders); bullet-level trim drops bullets in the
order guest -> group node -> you -> other edges, with the speaker
bullet as the MUST-tier floor (the speaker's own current activity is
the load-bearing slice).

Implementation chose Option B from the polish plan: pre-truncate the
bullets list at trim time before _build_activity_block runs, rather
than introduce a granular tier mode in the trim machinery. Rationale
documented in code; the existing block-level trim ladder gains a
single new toggle (include_you_activity) and the SHOULD-tier
guest_activity_block is gone.

Tests:
- test_single_activities_block_with_three_bullets_when_3_entities:
  exactly one ACTIVITIES: header with all three entity bullets.
- test_tight_budget_drops_guest_activity_bullet_first: speaker bullet
  survives, guest bullet absent under tight budget.
- Existing test_assemble_with_tight_budget_drops_guest_activity_first
  still passes (asserts on bullet absence, not block-header absence).

tests/test_prompt.py

@@ -514,6 +514,66 @@ def test_speaker_is_host_uses_host_witness_role(tmp_path, monkeypatch):
     assert captured["witness_role"] == "host"
 
 
+def test_single_activities_block_with_three_bullets_when_3_entities(tmp_path):
+    """T71.2: with you + host + guest present, the assembled prompt
+    contains exactly ONE ``ACTIVITIES:`` header and bullets for all
+    three entities (no duplicate header from the prior dual-block
+    rendering).
+    """
+    db = tmp_path / "t.db"
+    apply_migrations(db)
+    with open_db(db) as conn:
+        _seed_with_guest(conn)
+        msgs = assemble_narrative_prompt(
+            conn,
+            chat_id="chat_bot_a",
+            speaker_bot_id="bot_a",
+            recent_dialogue=[],
+            retrieved_memory_summaries=[],
+        )
+    body = msgs[0].content
+    # Exactly one ACTIVITIES: header.
+    assert body.count("ACTIVITIES:") == 1
+    # Bullets for all three entities (you=Sam, host=Aria, guest=Iris)
+    # — pin on the unique action verbs from the seed data.
+    assert "finishing emails" in body  # you bullet
+    assert "pretending to work" in body  # speaker (host) bullet
+    assert "smirking-distinctively" in body  # guest bullet
+
+
+def test_tight_budget_drops_guest_activity_bullet_first(tmp_path):
+    """T71.2: under tight budget the speaker bullet survives but the
+    guest activity bullet is the first ACTIVITIES: bullet to drop. The
+    block as a whole stays present (it's MUST-tier); only its body
+    contracts.
+    """
+    db = tmp_path / "t.db"
+    apply_migrations(db)
+    with open_db(db) as conn:
+        _seed_with_guest(conn)
+        dialogue = [
+            {"speaker": "you", "text": "line-16 hi there"},
+            {"speaker": "bot_a", "text": "line-17 hey"},
+            {"speaker": "you", "text": "line-18 quiet night"},
+            {"speaker": "bot_a", "text": "line-19 indeed"},
+        ]
+        msgs = assemble_narrative_prompt(
+            conn,
+            chat_id="chat_bot_a",
+            speaker_bot_id="bot_a",
+            recent_dialogue=dialogue,
+            retrieved_memory_summaries=[],
+            budget_soft=250,
+            budget_hard=340,
+        )
+    body = msgs[0].content
+    # Speaker bullet survives (MUST-tier floor).
+    assert "pretending to work" in body
+    assert "ACTIVITIES:" in body
+    # Guest bullet is dropped first under budget pressure.
+    assert "smirking-distinctively" not in body
+
+
 def test_assemble_with_tight_budget_drops_guest_activity_first(tmp_path):
     """Under tight budget MUST blocks survive but SHOULD-tier guest
     activity is dropped first."""