fix(agent_writer): normalize missing trailing newline before child splice (#72)

[MarcoPorcellato]

Summary

• Normalize source Markdown in append_child_to_node when the file lacks a final newline, so agent-write inserts a new child on its own line instead of concatenating onto the last row.
• Add regression test test_append_child_to_node_without_trailing_source_newline.
• Document splice behavior in docs/ARCHITECTURE.md §3.4.

Closes #72

Test plan

• make all (Ruff, Mypy, pytest)
• New test covers file without trailing \n on last line

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