citations: consolidate prompts, retire eager path, refresh ADR

Rewrite the main-agent citation contract to a single [n] channel and sync
the orphaned system_prompt_composer surface to match; drop stale
[citation:chunk_id] / <chunk_index> references from dynamic_context and
provider hints. Reuse the shared hybrid search in the deliverables report
(citations omitted for now) and delete the orphaned report KB helper.
Remove the dead eager KnowledgePriorityMiddleware wiring (knowledge_priority
+ stack) and its legacy browse test. Update ADR 0001 to reflect the cutover.

surfsense_backend/tests/unit/agents/new_chat/prompts/test_composer.py

@@ -86,9 +86,10 @@ class TestCompose:
         # Tools
         assert "<tools>" in prompt
         assert "</tools>" in prompt
-        # Citations on by default
+        # Citations on by default — the [n] / <retrieved_context> contract
         assert "<citation_instructions>" in prompt
-        assert "[citation:chunk_id]" in prompt
+        assert "<retrieved_context>" in prompt
+        assert "[1][2]" in prompt
 
     def test_team_visibility_uses_team_variants(self, fixed_today: datetime) -> None:
         prompt = compose_system_prompt(
@@ -116,9 +117,9 @@ class TestCompose:
     def test_citations_disabled_swaps_block(self, fixed_today: datetime) -> None:
         prompt_on = compose_system_prompt(today=fixed_today, citations_enabled=True)
         prompt_off = compose_system_prompt(today=fixed_today, citations_enabled=False)
-        assert "Citations are DISABLED" in prompt_off
-        assert "Citations are DISABLED" not in prompt_on
-        assert "[citation:chunk_id]" in prompt_on
+        assert "Citation markers are **disabled**" in prompt_off
+        assert "Citation markers are **disabled**" not in prompt_on
+        assert "<retrieved_context>" in prompt_on
 
     def test_enabled_tool_filter_only_includes_listed_tools(
         self, fixed_today: datetime