#174 Remove the pytest plugin, recommend inline-snapshot instead

Summary

• Remove devtools/pytest_plugin.py (insert_assert()), its tests, the pytest11 entry point and the black dev dependency it needed
• Add a "Snapshot testing" section to the usage docs recommending inline-snapshot, which does the same job better and is actively maintained
• Drop the docs claim that a pytest fixture installs debug into builtins, since that fixture lived in the plugin

Breaking change for anyone using insert_assert, so the next release should be 0.13.0.

Test plan

• ruff, basedpyright clean
• 106 passed on 3.14
• coverage 93.79%, above the smokeshow threshold
• make docs strict build passes

🤖 Generated with Claude Code

https://claude.ai/code/session_0125Pi5iurTnuhZ5cZ58xcNn

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Summary by cubic

Removes the bundled pytest plugin (insert_assert()) and recommends inline-snapshot instead, which does the same job better and is actively maintained.

• Removes the plugin, its tests, the pytest11 entry point, and the black dependency it needed.
• Adds a "Snapshot testing" section to the usage docs.
• Drops the claim that a pytest fixture installs debug into builtins.

This is a breaking change for anyone using insert_assert, so the next release should be 0.13.0.