Add edit_file tool — Claude Code Edit semantics

Literal string replacement with uniqueness check. Fails if old_string
is not found or matches multiple times (unless replace_all=true).

Matches the Claude Code harness Edit tool so sibling lmcp clients get
the same behaviour they already expect for in-place patches.

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

server.lua

@@ -194,6 +194,60 @@ server:tool("write_file", "Write content to a file.", {
     return string.format("Written %d bytes to %s", #a.content, a.path)
 end)
 
+server:tool("edit_file", "Replace exact text in a file (literal match). Fails unless old_string is unique, unless replace_all=true.", {
+    type = "object",
+    properties = {
+        path = { type = "string", description = "Path to file" },
+        old_string = { type = "string", description = "Exact text to replace (literal, no regex)" },
+        new_string = { type = "string", description = "Replacement text" },
+        replace_all = { type = "boolean", description = "Replace every occurrence (default: false)", default = false },
+    },
+    required = { "path", "old_string", "new_string" },
+}, function(a)
+    if type(a.path) ~= "string" or a.path == "" then return "Error: path required" end
+    if type(a.old_string) ~= "string" then return "Error: old_string required" end
+    if type(a.new_string) ~= "string" then return "Error: new_string required" end
+    if a.old_string == "" then return "Error: old_string cannot be empty" end
+    if a.old_string == a.new_string then return "Error: new_string must differ from old_string" end
+
+    local f = io.open(a.path, 'rb')
+    if not f then return "Error: could not read " .. a.path end
+    local content = f:read('*a'); f:close()
+
+    local count, pos = 0, 1
+    while pos <= #content do
+        local i = content:find(a.old_string, pos, true)
+        if not i then break end
+        count = count + 1
+        pos = i + #a.old_string
+    end
+
+    if count == 0 then
+        return "Error: old_string not found in " .. a.path
+    end
+    if count > 1 and not a.replace_all then
+        return string.format("Error: old_string matches %d times in %s (use replace_all=true or provide more surrounding context to disambiguate)", count, a.path)
+    end
+
+    local parts, p, replaced = {}, 1, 0
+    while true do
+        local i = content:find(a.old_string, p, true)
+        if not i then break end
+        parts[#parts+1] = content:sub(p, i-1)
+        parts[#parts+1] = a.new_string
+        p = i + #a.old_string
+        replaced = replaced + 1
+        if not a.replace_all then break end
+    end
+    parts[#parts+1] = content:sub(p)
+
+    local w = io.open(a.path, 'wb')
+    if not w then return "Error: could not write " .. a.path end
+    w:write(table.concat(parts)); w:close()
+
+    return string.format("Edited %s: %d replacement(s)", a.path, replaced)
+end)
+
 server:tool("list_dir", "List directory contents.", {
     type = "object",
     properties = { path = { type = "string", default = "." } },