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>

src/jarvis/reply/engine.py

@@ -9,7 +9,11 @@ import os
 from typing import Optional, TYPE_CHECKING
 
 from ..utils.redact import redact
-from ..system_prompt import build_system_prompt, reply_language_directive
+from ..system_prompt import (
+    build_system_prompt,
+    load_agent_instructions,
+    reply_language_directive,
+)
 from ..tools.registry import run_tool_with_retries, generate_tools_description, generate_tools_json_schema, BUILTIN_TOOLS
 from ..tools.builtin.stop import STOP_SIGNAL
 from ..debug import debug_log
@@ -1702,6 +1706,10 @@ def run_reply_engine(db: "Database", cfg, tts: Optional[Any],
     # the directive used the config value made the two contradict each other.
     _output_language = _resolve_output_language()
     _persona_prompt = build_system_prompt(_assistant_name, _output_language)
+    # File-based operator instructions: every *.md in AGENTS_DIR (default
+    # /app/agents, bind-mounted from ./agents). Read once per turn so edits in
+    # the folder apply on the next reply without a restart; fail-open to "".
+    _agent_instructions = load_agent_instructions()
 
     def _build_initial_system_message() -> str:
         guidance = [_persona_prompt.strip()]
@@ -1810,6 +1818,12 @@ def run_reply_engine(db: "Database", cfg, tts: Optional[Any],
         if _user_instructions:
             guidance.append("Additional instructions from the operator:\n" + _user_instructions)
 
+        # File-based operator instructions: the concatenated agents/*.md content
+        # resolved once above. Same framing/placement as the settings-UI field
+        # so both are treated as authoritative operator guidance.
+        if _agent_instructions:
+            guidance.append("Additional instructions from the operator:\n" + _agent_instructions)
+
         # Recency reinforcement: repeat the language lock at the very END too.
         # In a ~5k-token prompt the front-placed rule gets "lost in the middle";
         # bigger models (qwen2.5:7b) otherwise leak Chinese/Cyrillic mid-reply.