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>

docs/llm_contexts.md

@@ -13,6 +13,7 @@ Every distinct LLM call in Jarvis, what feeds it, what consumes it, and how it i
   - Redacted user query
   - Recent dialogue (last 5 minutes), including in-loop tool-call + tool-role messages from prior replies within the active conversation (tool carryover, `DialogueMemory.record_tool_turn` / `get_recent_turns_with_tools` in [src/jarvis/memory/conversation.py](src/jarvis/memory/conversation.py); per-prompt cap via `cfg.tool_carryover_max_turns` / `tool_carryover_per_entry_chars`; storage cap `_tool_turns_max_storage = 16`; cleared on `stop` signal AND on new-conversation entry; UNTRUSTED WEB EXTRACT fence markers preserved on truncation; both `content` and `tool_calls[*].function.arguments` scrubbed on write)
   - Unified system prompt from [src/jarvis/system_prompt.py](src/jarvis/system_prompt.py) + ASR note + tool-protocol guidance. Reply language is resolved by `reply_language_directive(lang, cfg.tts_engine)` where `lang = _resolve_output_language()` — the single source of truth that prefers the settings-web UI value (config JSON `output_language`) over the compose `OUTPUT_LANGUAGE` env, so changing the language in the settings page takes effect. An explicit lock wins (forces "reply only in `<language>`", also forbidding other scripts so small models stop leaking trailing CJK/Hanja); else a Piper/Chatterbox TTS forces English (English-only voices); else (multilingual TTS, no lock) the assistant replies in the user's own language. The directive is inserted near the FRONT of the guidance list so a small model gives it primacy, and the SAME resolved `lang` feeds `build_system_prompt()`, which rewrites the persona's "in the user's language" clause to the locked language so the persona cannot contradict the directive (previously the persona read the raw env while the directive read the config value, so a settings-UI change was honoured by one and ignored by the other). Gated in `_build_initial_system_message()` at [engine.py](src/jarvis/reply/engine.py).
+  - **Operator instructions** (two sources, both framed "Additional instructions from the operator:" and appended near the end of the guidance list): the settings-UI `llm_instructions` config field, and every `*.md` file in `AGENTS_DIR` (default `/app/agents`, bind-mounted from `./agents`). The file-based set is read once per turn by `load_agent_instructions()` in [src/jarvis/system_prompt.py](src/jarvis/system_prompt.py) and concatenated in filename order, so dropping/editing a `.md` applies on the next reply with no rebuild/restart; fail-open to `""` when the folder is absent/empty/unreadable.
   - **Warm profile block** (query-agnostic User + Directives excerpt from the knowledge graph, composed by `build_warm_profile()` / `format_warm_profile_block()` in [src/jarvis/memory/graph_ops.py](src/jarvis/memory/graph_ops.py) at Step 3.5 of `reply()`; no LLM call, pure SQLite read; injected unconditionally so personalisation is the default; result cached in `DialogueMemory._hot_cache` under `DialogueMemory.WARM_PROFILE_CACHE_KEY` for the lifetime of the active conversation. Invalidated on `stop`, on new-conversation entry, AND on User/Directives graph mutations via the listener registered in [src/jarvis/daemon.py](src/jarvis/daemon.py) against `register_graph_mutation_listener` in [src/jarvis/memory/graph.py](src/jarvis/memory/graph.py); World-branch writes are ignored)
   - Digested memory enrichment (optional, see #4)
   - Time + location context (re-injected each turn)