feat: human-readable path formatting in NodeTypeError messages

[nathanjmcdougall]

Summary

Replace raw tuple repr in NodeTypeError messages with human-readable path strings.

Before: Value at ('repos', 0, 'steps') is not a list
After: Value at repos > 0 > steps is not a list

Changes

• New src/yamltrip/_display.py module with format_path helper
• Updated 3 call sites in document.py: remove_from_list, ensure_in_list, find_index
• Tests for format_path and error message formatting

Closes #37

[Copilot]

Pull request overview

This PR improves NodeTypeError usability by replacing raw Python tuple representations of key paths (e.g., ('repos', 0, 'steps')) with a human-readable path string (e.g., repos > 0 > steps) in error messages emitted from Document list operations.

Changes:

• Added yamltrip._display.format_path() to format (str|int, ...) key tuples into a > 0 > b path strings (with <root> for ()).
• Updated Document.remove_from_list, Document.ensure_in_list, and Document.find_index to use format_path(keys) in their NodeTypeError messages.
• Added focused unit tests for format_path and for the updated NodeTypeError message formatting.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/yamltrip/_display.py	Introduces format_path() helper for human-readable key-path formatting.
src/yamltrip/document.py	Switches NodeTypeError message interpolation to use format_path() at three call sites.
tests/test_display.py	Adds unit tests covering format_path() behavior (single key, multi-key, int index, root).
tests/test_document.py	Adds assertions that list-operation NodeTypeError messages use the new path formatting.
doc/specs/2026-05-22-error-message-tuple-repr-design.md	Documents the problem, chosen formatting, and scope/testing plan.
.importlinter	Updates import-linter configuration to account for the new internal _display module.

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