Fix EditMemoryTool: remove ReplaceContentTool dependency

[EarthmanWeb]

Problem

EditMemoryTool.apply() delegates to ReplaceContentTool to perform
its text replacement. This creates an indirect dependency on the LSP
layer: ReplaceContentTool routes through the language server to
locate and modify files, which is unnecessary and incorrect for memory
files — memory files live in .serena/memories/ and are not part of
the indexed project source.

In contexts where the LSP is not active or the memory path is outside
the project root, this causes EditMemoryTool to fail entirely.

Fix

Replaced the ReplaceContentTool delegation in memory_tools.py with
direct file I/O using Python's standard re module:

• Reads the memory file directly via _find_memory()
• Performs literal or regex replacement inline
• Writes the result back — no LSP involvement

Behaviour is identical from the caller's perspective: supports both
"literal" and "regex" modes, returns a descriptive error if the
needle/pattern is not found.

Example

Before (broken in non-LSP contexts):

# EditMemoryTool.apply() called ReplaceContentTool which required an
# active language server and a path relative to the project root.
# Memory files outside the project root would raise an exception.

After:

memory_path = self.memories_manager._find_memory(memory_name)
content = memory_path.read_text(encoding="utf-8")
new_content = content.replace(needle, repl, 1)  # literal mode
memory_path.write_text(new_content, encoding="utf-8")

[opcode81]

This is already changed in #1091 (to be merged soon), where we factored out the replacement logic to a separate utility class.