Release v0.3.1: version bump

[devin-ai-integration]

Summary

Version bump for the v0.3.1 release: mod_version and ide-plugin pluginVersion → 0.3.1. Release v0.3.1 is already published at tag on this head (d9a01a4), matching the established flow.

Link to Devin session: https://app.devin.ai/sessions/f2c11399d60948a394f9836472375e2f
Open in Devin Desktop: https://app.devin.ai/desktop/session/f2c11399d60948a394f9836472375e2f?variant=devin
Requested by: @CommunityPoke

[devin-ai-integration]

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