fix: error on explicit missing config path; update env.example; add README config docs

- config.rs: NoxaConfig::load() now exits with an error when an explicit path
  (via --config arg or NOXA_CONFIG env var) does not exist; implicit ./config.json
  missing still silently returns default
- Updated test: test_load_missing_file_returns_default replaced with
  test_load_implicit_missing_file_returns_default (tests None path, not explicit)
- env.example: replaced fat legacy content with slim secrets-only template
  matching .env.example; deleted redundant .env.example
- README.md: replaced bare env-var table with full Configuration section
  covering config.json workflow, precedence, NOXA_CONFIG override, and bool
  flag limitation

env.example

@@ -1,46 +1,17 @@
-# ============================================
-# Noxa Configuration
-# Copy to .env and fill in your values
-# ============================================
+# Secrets and URLs only — everything else goes in config.json
+# See config.example.json for the full list of configurable defaults.
 
-# --- LLM Providers ---
+# Cloud API key (required for --cloud / --research)
+NOXA_API_KEY=
 
-# Gemini CLI (primary provider — requires `gemini` binary on PATH)
-# GEMINI_MODEL=gemini-2.5-pro   # defaults to gemini-2.5-pro
+# Single proxy URL (or use NOXA_PROXY_FILE for pool rotation)
+NOXA_PROXY=
 
-# Ollama (fallback; local inference)
-OLLAMA_HOST=http://localhost:11434
-OLLAMA_MODEL=qwen3:8b
+# Webhook URL for completion notifications
+NOXA_WEBHOOK_URL=
 
-# OpenAI (optional cloud fallback)
-# OPENAI_API_KEY   — set your OpenAI key
-# OPENAI_BASE_URL  — defaults to https://api.openai.com/v1
-# OPENAI_MODEL     — defaults to gpt-4o-mini
+# LLM base URL (Ollama or OpenAI-compatible endpoint)
+NOXA_LLM_BASE_URL=
 
-# Anthropic (optional cloud fallback)
-# ANTHROPIC_API_KEY — set your Anthropic key
-# ANTHROPIC_MODEL  — defaults to claude-sonnet-4-20250514
-
-# --- Proxy ---
-
-# Single proxy
-# NOXA_PROXY=http://user:pass@host:port
-
-# Proxy file (one per line: host:port:user:pass)
-# NOXA_PROXY_FILE=/path/to/proxies.txt
-
-# --- Server (noxa-server only) ---
-# NOXA_PORT=3000
-# NOXA_HOST=0.0.0.0
-# NOXA_AUTH_KEY=your-auth-key
-# NOXA_MAX_CONCURRENCY=50
-# NOXA_JOB_TTL_SECS=3600
-# NOXA_MAX_JOBS=100
-
-# --- CLI LLM overrides ---
-# NOXA_LLM_PROVIDER=ollama
-# NOXA_LLM_MODEL=qwen3:8b
-# NOXA_LLM_BASE_URL=http://localhost:11434
-
-# --- Logging ---
-# NOXA_LOG=info
+# Optional: path to a non-default config file (default: ./config.json)
+# NOXA_CONFIG=/path/to/my-config.json