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>

agents/example.md.sample

@@ -0,0 +1,15 @@
+# Operator instruction file (example)
+#
+# HOW TO USE: copy or rename this file to anything ending in `.md`
+# (e.g. `rules.md`). Every `*.md` in this folder is appended to the assistant's
+# main reply system prompt, in filename order — use number prefixes like
+# `00-tone.md`, `10-rules.md` to control ordering. Edits take effect on the
+# NEXT reply; no rebuild or restart is needed (the folder is read per turn).
+#
+# Files ending in `.sample` (like this one) are ignored, so this template never
+# affects replies until you rename it to `*.md`.
+#
+# Everything below a heading is treated as plain instruction text for the LLM.
+
+Always keep replies under two sentences.
+When the user asks about deployment, mention the relevant docker compose command.