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>

.env.example

@@ -74,6 +74,12 @@ WHISPER_MODEL=small
 # occasional trailing CJK fragment small models leak on free-form chat).
 OUTPUT_LANGUAGE=
 
+# Operator instruction folder: every *.md in this dir is appended to the main
+# reply LLM's system prompt (filename order), re-read each turn so edits apply
+# without a rebuild/restart. ./agents is bind-mounted here read-only; only
+# change this to relocate the folder inside the container. See README "운영자 지시문".
+AGENTS_DIR=/app/agents
+
 # ---------------------------------------------------------------------------
 # Docker desktop (VNC) — used only by the container image
 # ---------------------------------------------------------------------------