docs: Add server-prompt labeled panel for built-in instructions

Refactor system-prompt CSS into shared base for two panel variants:
- .system-prompt — "SYSTEM PROMPT" label for user-authored fragments
- .server-prompt — "BUILT-IN SERVER PROMPT" for libtmux-mcp's own instructions

Applies server-prompt to the auto-generated instructions block on
the prompting guide page.

Author: tony

docs/_static/css/custom.css

@@ -246,17 +246,20 @@ div.admonition.agent-thought > p:last-child {
   margin-bottom: 0;
 }
 
-/* ── System prompt fragments ─────────────────────────────
- * Copyable instruction prose for AGENTS.md / CLAUDE.md.
- * Internal label row, softer dark panel, prose wrapping.
+/* ── Labeled code panels ─────────────────────────────────
+ * Copyable prose in labeled dark panels. Two variants:
+ * .system-prompt — user-authored fragments for AGENTS.md
+ * .server-prompt — libtmux-mcp's built-in instructions
  * Keeps sphinx-copybutton via .highlight > pre selector.
  * ────────────────────────────────────────────────────────── */
-div.system-prompt {
+div.system-prompt,
+div.server-prompt {
   margin: 1.25rem 0;
   position: relative;
 }
 
-div.system-prompt > div.highlight {
+div.system-prompt > div.highlight,
+div.server-prompt > div.highlight {
   background: #1f2329 !important;
   border: 1px solid color-mix(in srgb, var(--color-link) 20%, transparent);
   border-left: 3px solid var(--color-link);
@@ -265,17 +268,18 @@ div.system-prompt > div.highlight {
   padding-top: 1.3rem;
 }
 
-div.system-prompt > div.highlight > pre {
+div.system-prompt > div.highlight > pre,
+div.server-prompt > div.highlight > pre {
   background: transparent;
   font-size: 13px;
   line-height: 1.6;
   white-space: pre-wrap;
   word-break: break-word;
 }
 
-/* Internal label — sits inside the block, not floating above */
-div.system-prompt > div.highlight::before {
-  content: "System prompt";
+/* Internal label — quiet uppercase whisper */
+div.system-prompt > div.highlight::before,
+div.server-prompt > div.highlight::before {
   position: absolute;
   top: 0.45rem;
   left: 0.85rem;
@@ -291,8 +295,17 @@ div.system-prompt > div.highlight::before {
   opacity: 0.8;
 }
 
+div.system-prompt > div.highlight::before {
+  content: "System prompt";
+}
+
+div.server-prompt > div.highlight::before {
+  content: "Built-in server prompt";
+}
+
 /* Fix copy button background to match panel */
-div.system-prompt .copybtn {
+div.system-prompt .copybtn,
+div.server-prompt .copybtn {
   background: #1f2329 !important;
 }
 

docs/topics/prompting.md

@@ -8,7 +8,9 @@ How to write effective instructions for AI agents using libtmux-mcp.
 
 Every MCP client receives these instructions when connecting to the libtmux-mcp server. You do not need to repeat this information — the agent already knows it.
 
-```text
+```{code-block} text
+:class: server-prompt
+
 libtmux MCP server for programmatic tmux control. tmux hierarchy:
 Server > Session > Window > Pane. Use pane_id (e.g. '%1') as the
 preferred targeting method - it is globally unique within a tmux server.