test(agent_writer): regression for append_child without trailing source newline

[MarcoPorcellato]

Summary

Add a pytest that reproduces #72: append_child_to_node must not splice a new bullet onto the previous line when the source .md file lacks a final newline.

Clean Architecture lens

Aspect	Assessment
Ring	Tests (driver contract) validating adapter agent_writer.py
SOLID	LSP — file shape variants (with/without trailing \n) should behave identically

Task

1. Extend tests/test_agent_writer.py with a case where page_path.write_text("- parent\n - existing", encoding="utf-8") (no trailing newline).
2. Assert disk lines and in-memory parent.children after append_child_to_node.
3. Pair with fix in fix(agent_writer): append_child corrupts markdown when source file lacks trailing newline #72.

Context

Local code study wave 11. See docs/GOOD_FIRST_ISSUES.md GFI-34.

[vbhavh]

/assign

[MarcoPorcellato]

Thanks for taking the issue