renderer: tool-call begin/end frames

Phase 2 commit #4 per docs/PHASE2.md §12. Adds M.tool_call_begin(name, args)
and M.tool_call_end(content, is_error) for visual parity with the existing
exec_begin/exec_end frame.

Visual cadence:
  ─── tool: <name (cyan)> ───
  <args, dim, truncated at 200 chars; omitted if empty/"{}">
  <content>
  ─── ok ───            (dim, success)
  ─── error ───         (red status word inside dim rule, on is_error=true)

Same rule glyph (━) and ANSI palette as the exec frame so the user reads
tool dispatch and shell dispatch the same way.

Smoke-tested all five shapes: success with args / empty args / error /
long args truncated / empty content.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

renderer.lua

@@ -76,4 +76,35 @@ function M.assistant_flush()
     stream_buf = nil
 end
 
+-- Phase 2: MCP tool-call frame. Visual parity with the exec_begin/exec_end
+-- frame so the user reads tool dispatch and shell dispatch the same way.
+-- tool_call_begin renders the top rule + (optionally) the args as a dim
+-- preview; tool_call_end renders the result content followed by a status
+-- rule. Status is "ok" (dim) by default; "error" (red) if is_error is true.
+-- See docs/PHASE2.md §3 renderer.lua row + §4 Tool invocation.
+
+function M.tool_call_begin(name, args)
+    emit(A.dim, "─── tool: ", A.reset,
+         A.cyan, name, A.reset,
+         A.dim, " ───", A.reset, "\n")
+    if args and args ~= "" and args ~= "{}" then
+        local shown = (#args <= 200) and args or (args:sub(1, 197) .. "...")
+        emit(A.dim, shown, A.reset, "\n")
+    end
+end
+
+function M.tool_call_end(content, is_error)
+    if content and content ~= "" then
+        emit(content)
+        if not content:match("\n$") then emit("\n") end
+    end
+    if is_error then
+        emit(A.dim, "─── ", A.reset,
+             A.red, "error", A.reset,
+             A.dim, " ───", A.reset, "\n")
+    else
+        emit(A.dim, "─── ok ───", A.reset, "\n")
+    end
+end
+
 return M