docs(configuration): Use Sphinx {envvar} directive and role for env vars

why: Sphinx has a built-in envvar system that creates proper anchors,
index entries, and inline-code-styled cross-references — better than
manual labels with {ref}.
what:
- Replace manual (LABEL)= + ### headings with {envvar} directives
- Replace {ref}`LIBTMUX_SAFETY` with {envvar}`LIBTMUX_SAFETY` in
  safety page and glossary
- Renders as code-styled links (like {ref} for tools)
- Auto-generates index entries for each env var

Author: tony

docs/configuration.md

@@ -6,36 +6,32 @@ Runtime configuration for the libtmux-mcp server. For MCP client setup, see {ref
 
 ## Environment variables
 
-(LIBTMUX_SOCKET)=
-
-### `LIBTMUX_SOCKET`
+```{envvar} LIBTMUX_SOCKET
+```
 
 tmux socket name (`-L`). Isolates the MCP server to a specific tmux socket.
 
 - **Type:** string
 - **Default:** (none — uses the default tmux socket)
 
-(LIBTMUX_SOCKET_PATH)=
-
-### `LIBTMUX_SOCKET_PATH`
+```{envvar} LIBTMUX_SOCKET_PATH
+```
 
 tmux socket path (`-S`). Alternative to socket name for custom socket locations.
 
 - **Type:** string
 - **Default:** (none)
 
-(LIBTMUX_TMUX_BIN)=
-
-### `LIBTMUX_TMUX_BIN`
+```{envvar} LIBTMUX_TMUX_BIN
+```
 
 Path to tmux binary. Useful for testing with different tmux versions.
 
 - **Type:** string
 - **Default:** `tmux`
 
-(LIBTMUX_SAFETY)=
-
-### `LIBTMUX_SAFETY`
+```{envvar} LIBTMUX_SAFETY
+```
 
 Safety tier controlling which tools are available. See {ref}`safety`.
 
@@ -64,7 +60,7 @@ Set environment variables in your MCP client config:
 
 ## Socket isolation
 
-By default, the MCP server connects to the default tmux socket. Set {ref}`LIBTMUX_SOCKET` to isolate AI agent activity from your personal tmux sessions:
+By default, the MCP server connects to the default tmux socket. Set {envvar}`LIBTMUX_SOCKET` to isolate AI agent activity from your personal tmux sessions:
 
 ```json
 "env": { "LIBTMUX_SOCKET": "ai_workspace" }
@@ -74,4 +70,4 @@ The agent will only see sessions on the `ai_workspace` socket, not your personal
 
 ## All tools accept `socket_name`
 
-Every tool accepts an optional `socket_name` parameter that overrides {ref}`LIBTMUX_SOCKET` for that call. This allows agents to work across multiple tmux servers in a single session.
+Every tool accepts an optional `socket_name` parameter that overrides {envvar}`LIBTMUX_SOCKET` for that call. This allows agents to work across multiple tmux servers in a single session.

docs/glossary.md

@@ -29,7 +29,7 @@ Pane
     A tmux pane within a window. A pseudoterminal that runs a single process. Has an ID (e.g. `%1`) that is globally unique within a server.
 
 Safety tier
-    A level controlling which MCP tools are available: `readonly`, `mutating`, or `destructive`. Set via the {ref}`LIBTMUX_SAFETY` env var.
+    A level controlling which MCP tools are available: `readonly`, `mutating`, or `destructive`. Set via the {envvar}`LIBTMUX_SAFETY` env var.
 
 Socket
     The Unix socket used to communicate with a tmux server. Can be specified by name (`-L`) or path (`-S`).

docs/topics/safety.md

@@ -14,7 +14,7 @@ libtmux-mcp uses a three-tier safety system to control which tools are available
 
 ## Configuration
 
-Set the safety tier via the {ref}`LIBTMUX_SAFETY` environment variable:
+Set the safety tier via the {envvar}`LIBTMUX_SAFETY` environment variable:
 
 ```json
 {