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>

docker-compose.yml

@@ -97,6 +97,10 @@ services:
       BROWSER_CONTROL_BIND: ${BROWSER_CONTROL_BIND:-0.0.0.0}
       BROWSER_CONTROL_PORT: ${BROWSER_CONTROL_PORT:-8777}
       BROWSER_CONTROL_URL: ${BROWSER_CONTROL_URL:-}
+      # Folder of operator *.md instruction files appended to the main reply
+      # LLM's system prompt. Bind-mounted from ./agents below; override only to
+      # relocate it inside the container.
+      AGENTS_DIR: ${AGENTS_DIR:-/app/agents}
     # No hard depends_on ollama: a browser-host (`docker compose up -d javis`)
     # must NOT pull in Ollama. Full/bot layouts start it with a plain
     # `docker compose up -d` (all services); the bridge tolerates Ollama warming
@@ -149,6 +153,11 @@ services:
       # If unseeded, the path fail-opens to the DDG/Brave cascade and the
       # entrypoint logs a warning. Only consumed when GEMINI_AUTH=oauth.
       - ${GEMINI_OAUTH_DIR:-./docker/gemini-oauth}:/root/.gemini
+      # Operator instruction files. Every *.md here is appended to the main
+      # reply LLM's system prompt (filename order), read per turn so edits apply
+      # on the next reply without a rebuild/restart. Read-only; a project-
+      # relative path resolves identically on Linux and Windows Docker Desktop.
+      - ./agents:/app/agents:ro
 
 volumes:
   ollama_models: