docs: Add configuration reference page

why: Configuration was scattered across quickstart and about. Users
looking for env var docs shouldn't wade through installation steps.
what:
- Create docs/configuration.md with env var table
- Include type, default, and description for each variable
- Document socket isolation pattern with example
- Note per-tool socket_name override

Author: tony

docs/configuration.md

@@ -0,0 +1,45 @@
+(configuration)=
+
+# Configuration
+
+Runtime configuration for the libtmux-mcp server. For MCP client setup, see {ref}`clients`.
+
+## Environment variables
+
+| Variable | Type | Default | Description |
+|----------|------|---------|-------------|
+| `LIBTMUX_SOCKET` | string | (none) | tmux socket name (`-L`). Isolates the MCP server to a specific tmux socket. |
+| `LIBTMUX_SOCKET_PATH` | string | (none) | tmux socket path (`-S`). Alternative to socket name for custom socket locations. |
+| `LIBTMUX_TMUX_BIN` | string | `tmux` | Path to tmux binary. Useful for testing with different tmux versions. |
+| `LIBTMUX_SAFETY` | string | `mutating` | Safety tier: `readonly`, `mutating`, or `destructive`. See {ref}`safety`. |
+
+Set environment variables in your MCP client config:
+
+```json
+{
+    "mcpServers": {
+        "libtmux": {
+            "command": "uvx",
+            "args": ["libtmux-mcp"],
+            "env": {
+                "LIBTMUX_SOCKET": "ai_workspace",
+                "LIBTMUX_SAFETY": "readonly"
+            }
+        }
+    }
+}
+```
+
+## Socket isolation
+
+By default, the MCP server connects to the default tmux socket. Set `LIBTMUX_SOCKET` to isolate AI agent activity from your personal tmux sessions:
+
+```json
+"env": { "LIBTMUX_SOCKET": "ai_workspace" }
+```
+
+The agent will only see sessions on the `ai_workspace` socket, not your personal sessions.
+
+## All tools accept `socket_name`
+
+Every tool accepts an optional `socket_name` parameter that overrides `LIBTMUX_SOCKET` for that call. This allows agents to work across multiple tmux servers in a single session.