docs: fallback providers + /background command documentation

* docs: comprehensive fallback providers documentation

- New dedicated page: user-guide/features/fallback-providers.md covering
  both primary model fallback and auxiliary task fallback systems
- Updated configuration.md with fallback_model config section
- Updated environment-variables.md noting fallback is config-only
- Fleshed out developer-guide/provider-runtime.md fallback section with
  internal architecture details (trigger points, activation flow, config flow)
- Added cross-reference from provider-routing.md distinguishing OpenRouter
  sub-provider routing from Hermes-level model fallback
- Added new page to sidebar under Integrations

* docs: comprehensive /background command documentation

- Added Background Sessions section to cli.md covering how it works
  (daemon threads, isolated sessions, config inheritance, Rich panel
  output, bell notification, concurrent tasks)
- Added Background Sessions section to messaging/index.md covering
  messaging-specific behavior (async execution, result delivery back
  to same chat, fire-and-forget pattern)
- Documented background_process_notifications config
  (all/result/error/off) in messaging docs and configuration.md
- Added HERMES_BACKGROUND_NOTIFICATIONS env var to reference page
- Fixed inconsistency in slash-commands.md: /background was listed as
  messaging-only but works in both CLI and messaging. Moved it to the
  'both surfaces' note.
- Expanded one-liner table descriptions with detail and cross-references

website/docs/user-guide/cli.md

@@ -259,6 +259,55 @@ compression:
 
 When compression triggers, middle turns are summarized while the first 3 and last 4 turns are always preserved.
 
+## Background Sessions
+
+Run a prompt in a separate background session while continuing to use the CLI for other work:
+
+```
+/background Analyze the logs in /var/log and summarize any errors from today
+```
+
+Hermes immediately confirms the task and gives you back the prompt:
+
+```
+🔄 Background task #1 started: "Analyze the logs in /var/log and summarize..."
+   Task ID: bg_143022_a1b2c3
+```
+
+### How It Works
+
+Each `/background` prompt spawns a **completely separate agent session** in a daemon thread:
+
+- **Isolated conversation** — the background agent has no knowledge of your current session's history. It receives only the prompt you provide.
+- **Same configuration** — the background agent inherits your model, provider, toolsets, reasoning settings, and fallback model from the current session.
+- **Non-blocking** — your foreground session stays fully interactive. You can chat, run commands, or even start more background tasks.
+- **Multiple tasks** — you can run several background tasks simultaneously. Each gets a numbered ID.
+
+### Results
+
+When a background task finishes, the result appears as a panel in your terminal:
+
+```
+╭─ ⚕ Hermes (background #1) ──────────────────────────────────╮
+│ Found 3 errors in syslog from today:                         │
+│ 1. OOM killer invoked at 03:22 — killed process nginx        │
+│ 2. Disk I/O error on /dev/sda1 at 07:15                      │
+│ 3. Failed SSH login attempts from 192.168.1.50 at 14:30      │
+╰──────────────────────────────────────────────────────────────╯
+```
+
+If the task fails, you'll see an error notification instead. If `display.bell_on_complete` is enabled in your config, the terminal bell rings when the task finishes.
+
+### Use Cases
+
+- **Long-running research** — "/background research the latest developments in quantum error correction" while you work on code
+- **File processing** — "/background analyze all Python files in this repo and list any security issues" while you continue a conversation
+- **Parallel investigations** — start multiple background tasks to explore different angles simultaneously
+
+:::info
+Background sessions do not appear in your main conversation history. They are standalone sessions with their own task ID (e.g., `bg_143022_a1b2c3`).
+:::
+
 ## Quiet Mode
 
 By default, the CLI runs in quiet mode which: