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/reference/slash-commands.md

@@ -31,7 +31,7 @@ Type `/` in the CLI to open the autocomplete menu. Built-in commands are case-in
 | `/title` | Set a title for the current session (usage: /title My Session Name) |
 | `/compress` | Manually compress conversation context (flush memories + summarize) |
 | `/rollback` | List or restore filesystem checkpoints (usage: /rollback [number]) |
-| `/background` | Run a prompt in the background (usage: /background <prompt>) |
+| `/background <prompt>` | Run a prompt in a separate background session. The agent processes your prompt independently — your current session stays free for other work. Results appear as a panel when the task finishes. See [CLI Background Sessions](/docs/user-guide/cli#background-sessions). |
 | `/plan [request]` | Load the bundled `plan` skill to write a markdown plan instead of executing the work. Plans are saved under `.hermes/plans/` relative to the active workspace/backend working directory. |
 
 ### Configuration
@@ -109,7 +109,7 @@ The messaging gateway supports the following built-in commands inside Telegram,
 | `/reasoning [level\|show\|hide]` | Change reasoning effort or toggle reasoning display. |
 | `/voice [on\|off\|tts\|join\|channel\|leave\|status]` | Control spoken replies in chat. `join`/`channel`/`leave` manage Discord voice-channel mode. |
 | `/rollback [number]` | List or restore filesystem checkpoints. |
-| `/background &lt;prompt&gt;` | Run a prompt in a separate background session. |
+| `/background <prompt>` | Run a prompt in a separate background session. Results are delivered back to the same chat when the task finishes. See [Messaging Background Sessions](/docs/user-guide/messaging/#background-sessions). |
 | `/plan [request]` | Load the bundled `plan` skill to write a markdown plan instead of executing the work. Plans are saved under `.hermes/plans/` relative to the active workspace/backend working directory. |
 | `/reload-mcp` | Reload MCP servers from config. |
 | `/update` | Update Hermes Agent to the latest version. |
@@ -119,6 +119,6 @@ The messaging gateway supports the following built-in commands inside Telegram,
 ## Notes
 
 - `/skin`, `/tools`, `/toolsets`, `/config`, `/prompt`, `/cron`, `/skills`, `/platforms`, `/paste`, and `/verbose` are **CLI-only** commands.
-- `/status`, `/stop`, `/sethome`, `/resume`, `/background`, and `/update` are **messaging-only** commands.
-- `/voice`, `/reload-mcp`, and `/rollback` work in **both** the CLI and the messaging gateway.
+- `/status`, `/stop`, `/sethome`, `/resume`, and `/update` are **messaging-only** commands.
+- `/background`, `/voice`, `/reload-mcp`, and `/rollback` work in **both** the CLI and the messaging gateway.
 - `/voice join`, `/voice channel`, and `/voice leave` are only meaningful on Discord.