docs(tools[buffer]): restore UUID-namespacing + no-list_buffers rationale

Hoist the page-level intro that the mechanical split dropped. Original
prose lives at 970b000:docs/tools/buffers.md:1-13; the per-tool pages
inherit tool-scoped Use/Avoid blocks from autodoc but cannot carry
area-level design rationale, so buffer/index.md is the natural home.

Restores:
- UUID-scoped namespacing (libtmux_mcp_<32-hex>_<logical>) and the
  concurrent-agent collision-safety it provides.
- Deliberate absence of a list_buffers tool, because the user's OS
  clipboard often syncs into tmux paste buffers and generic
  enumeration would leak credentials.

Author: tony

docs/tools/buffer/index.md

@@ -1,6 +1,8 @@
 # Buffer tools
 
-Paste-buffer tools — stage, push, inspect, and delete MCP-namespaced tmux buffers for multi-line input.
+tmux paste buffers are a server-global namespace shared by every client on the same socket. The buffer tools in libtmux-mcp expose a narrow, agent-namespaced subset: every allocation gets a UUID-scoped name like `libtmux_mcp_<32-hex>_<logical>`, so concurrent agents (or parallel tool calls from one agent) cannot collide on each other's payloads.
+
+There is **no** `list_buffers` tool. The user's OS clipboard often syncs into tmux paste buffers, so a generic enumeration would leak passwords, tokens, and other private content the agent has no business reading. Callers track the buffers they own via the {tool}`load-buffer` returns.
 
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 :gutter: 2 2 3 3