Add Discord-native hybrid front-end for Jarvis (bot + bridge)

Transform isair/jarvis into a Discord-controlled voice assistant running on
the Ubuntu VNC desktop, keeping the mature ~39k-line Python brain intact.

- bot/ (Node + bun, discord.js): /자비스 slash commands (ephemeral),
  voice channel join + voice receive/playback, pluggable VNC screen broadcast
  (selfbot live / noVNC / screenshot)
- bridge/ (Python, Flask): wraps jarvis STT + run_reply_engine + Piper TTS
  behind a thin localhost HTTP API
- .env.example, scripts/ (start_bridge/start_bot/dev), README rewrite,
  docs/language-comparison.md and docs/vnc-xfce-setup.md

Language decision: hybrid (Python brain + Node/bun Discord layer) because
Discord blocks bot video; native screen broadcast only works via a Node
selfbot library.

tests/test_prompt_dump.py

@@ -0,0 +1,122 @@
+"""
+Unit tests for the opt-in prompt dump (src/jarvis/reply/prompt_dump.py).
+
+The dump exists because PR #232's harness evals cannot reproduce the live
+Possessor→"Under the Skin" confab. We need a way to capture the *exact*
+messages array the field hits so a deterministic eval can replay it.
+
+Tests focus on behaviours rather than internals:
+  * OFF by default (no file when env var unset).
+  * ON when env var is set — file lands in the expected directory with the
+    full messages array round-tripped.
+  * Failures during dump never propagate (diagnostics must not break replies).
+"""
+
+from __future__ import annotations
+
+import json
+import os
+from pathlib import Path
+from unittest.mock import patch
+
+import pytest
+
+from jarvis.reply import prompt_dump
+
+
+@pytest.fixture
+def tmp_home(tmp_path, monkeypatch):
+    """Redirect Path.home() so dumps land in a sandbox."""
+    monkeypatch.setattr(prompt_dump.Path, "home", lambda: tmp_path)
+    return tmp_path
+
+
+class TestGating:
+    def test_disabled_by_default(self, tmp_home, monkeypatch):
+        monkeypatch.delenv("JARVIS_DUMP_PROMPTS", raising=False)
+        result = prompt_dump.dump_reply_turn(
+            session_id="abc12345",
+            turn=1,
+            query="hi",
+            model="m",
+            messages=[{"role": "user", "content": "hi"}],
+            tools_schema=None,
+            use_text_tools=False,
+        )
+        assert result is None
+        # No files created.
+        assert not (tmp_home / ".local" / "share" / "jarvis" / "prompts").exists()
+
+    @pytest.mark.parametrize("value", ["1", "true", "YES", "on"])
+    def test_enabled_by_truthy_env_values(self, tmp_home, monkeypatch, value):
+        monkeypatch.setenv("JARVIS_DUMP_PROMPTS", value)
+        assert prompt_dump.is_enabled()
+
+    @pytest.mark.parametrize("value", ["", "0", "false", "no"])
+    def test_disabled_by_falsy_env_values(self, tmp_home, monkeypatch, value):
+        monkeypatch.setenv("JARVIS_DUMP_PROMPTS", value)
+        assert not prompt_dump.is_enabled()
+
+
+class TestDumpContents:
+    def test_writes_full_payload(self, tmp_home, monkeypatch, capsys):
+        monkeypatch.setenv("JARVIS_DUMP_PROMPTS", "1")
+        messages = [
+            {"role": "system", "content": "SYS"},
+            {"role": "user", "content": "Tell me about Possessor"},
+        ]
+        tools = [{"type": "function", "function": {"name": "webSearch"}}]
+        path = prompt_dump.dump_reply_turn(
+            session_id="deadbeef",
+            turn=2,
+            query="Tell me about Possessor",
+            model="gemma4:e2b",
+            messages=messages,
+            tools_schema=tools,
+            use_text_tools=False,
+            response={"message": {"content": "Under the Skin"}},
+        )
+        assert path is not None
+        assert path.exists()
+        assert path.parent == tmp_home / ".local" / "share" / "jarvis" / "prompts"
+        assert "deadbeef" in path.name
+        assert "t02" in path.name
+
+        payload = json.loads(path.read_text(encoding="utf-8"))
+        assert payload["session_id"] == "deadbeef"
+        assert payload["turn"] == 2
+        assert payload["query"] == "Tell me about Possessor"
+        assert payload["model"] == "gemma4:e2b"
+        assert payload["messages"] == messages
+        assert payload["tools_schema"] == tools
+        assert payload["use_text_tools"] is False
+        assert payload["response"]["message"]["content"] == "Under the Skin"
+
+        # User-visible line should mention the path so they know where to grab it.
+        out = capsys.readouterr().out
+        assert str(path) in out
+
+    def test_session_ids_are_unique_per_call(self):
+        ids = {prompt_dump.new_session_id() for _ in range(50)}
+        assert len(ids) == 50
+
+    def test_dump_failure_is_swallowed(self, tmp_home, monkeypatch):
+        """A broken serialiser must not propagate — prompts dump is a
+        diagnostic aid, never a hard dependency of reply generation."""
+        monkeypatch.setenv("JARVIS_DUMP_PROMPTS", "1")
+
+        class Unserialisable:
+            def __repr__(self):
+                raise RuntimeError("nope")
+
+        with patch("jarvis.reply.prompt_dump.json.dumps", side_effect=RuntimeError("boom")):
+            result = prompt_dump.dump_reply_turn(
+                session_id="abc",
+                turn=1,
+                query="q",
+                model="m",
+                messages=[],
+                tools_schema=None,
+                use_text_tools=False,
+            )
+        assert result is None  # swallowed, not raised