feat: load operator instructions from agents/*.md into the reply prompt

Drop Markdown files into an agents/ folder and their contents are appended to
the main reply LLM's system prompt, so an operator can extend the assistant's
rules/tone without code changes. Files are concatenated in filename order
(use 00-, 10- prefixes to control ordering) and re-read once per turn, so edits
apply on the next reply with no rebuild/restart. Fail-open: a missing, empty,
or unreadable folder yields no instructions and never breaks a reply.

- load_agent_instructions() in system_prompt.py (AGENTS_DIR env, default
  /app/agents); reads *.md only, skips blanks, ignores non-dir paths
- engine.py appends it alongside the existing settings-UI llm_instructions,
  under the same "Additional instructions from the operator:" framing
- docker-compose.yml bind-mounts ./agents:/app/agents:ro and sets AGENTS_DIR
- agents/example.md.sample starter template (.sample is not loaded)
- tests cover ordering, md-only filtering, blank-skip, env/arg resolution,
  and fail-open paths
- README, .env.example, docs/llm_contexts.md updated

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

tests/test_system_prompt.py

@@ -7,6 +7,7 @@ hardcoded to Jarvis.
 
 from jarvis.system_prompt import (
     build_system_prompt,
+    load_agent_instructions,
     output_language_directive,
     reply_language_directive,
     ENGLISH_ONLY_DIRECTIVE,
@@ -108,3 +109,57 @@ class TestReplyLanguageDirective:
     def test_lock_wins_even_with_multilingual_tts(self):
         directive = reply_language_directive("Korean", "melo")
         assert directive is not None and "Korean" in directive
+
+
+class TestLoadAgentInstructions:
+    """Operator can extend the reply LLM's system prompt by dropping *.md files
+    into an agents/ folder. The loader concatenates them in filename order and
+    fails open so a missing/empty/broken folder never breaks a reply."""
+
+    def test_missing_dir_returns_empty(self, tmp_path):
+        assert load_agent_instructions(str(tmp_path / "does-not-exist")) == ""
+
+    def test_empty_dir_returns_empty(self, tmp_path):
+        assert load_agent_instructions(str(tmp_path)) == ""
+
+    def test_reads_and_concatenates_single_file(self, tmp_path):
+        (tmp_path / "rules.md").write_text("Always be brief.", encoding="utf-8")
+        assert load_agent_instructions(str(tmp_path)) == "Always be brief."
+
+    def test_files_are_ordered_by_filename(self, tmp_path):
+        # Filename prefixes let the operator control ordering.
+        (tmp_path / "10-second.md").write_text("SECOND", encoding="utf-8")
+        (tmp_path / "00-first.md").write_text("FIRST", encoding="utf-8")
+        result = load_agent_instructions(str(tmp_path))
+        assert result.index("FIRST") < result.index("SECOND")
+
+    def test_only_md_files_are_read(self, tmp_path):
+        (tmp_path / "note.txt").write_text("IGNORE ME", encoding="utf-8")
+        (tmp_path / "use.md").write_text("USE ME", encoding="utf-8")
+        result = load_agent_instructions(str(tmp_path))
+        assert "USE ME" in result
+        assert "IGNORE ME" not in result
+
+    def test_blank_files_are_skipped(self, tmp_path):
+        (tmp_path / "blank.md").write_text("   \n  ", encoding="utf-8")
+        (tmp_path / "real.md").write_text("Real instruction.", encoding="utf-8")
+        assert load_agent_instructions(str(tmp_path)) == "Real instruction."
+
+    def test_env_var_is_used_when_no_arg(self, tmp_path, monkeypatch):
+        (tmp_path / "a.md").write_text("FROM ENV", encoding="utf-8")
+        monkeypatch.setenv("AGENTS_DIR", str(tmp_path))
+        assert load_agent_instructions() == "FROM ENV"
+
+    def test_explicit_arg_overrides_env(self, tmp_path, monkeypatch):
+        (tmp_path / "env.md").write_text("ENV", encoding="utf-8")
+        other = tmp_path / "other"
+        other.mkdir()
+        (other / "arg.md").write_text("ARG", encoding="utf-8")
+        monkeypatch.setenv("AGENTS_DIR", str(tmp_path))
+        assert load_agent_instructions(str(other)) == "ARG"
+
+    def test_a_file_path_instead_of_dir_returns_empty(self, tmp_path):
+        f = tmp_path / "file.md"
+        f.write_text("x", encoding="utf-8")
+        # Pointed at a file, not a directory → fail-open.
+        assert load_agent_instructions(str(f)) == ""