add grep tool result formatter

[LasmarKhalifa]

Closes #971

[LasmarKhalifa]

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• [PROTOTYPE] [WIP] Regex for parsing taskoutput results #984
• [PROTOTYPE] [WIP] REXML for parsing taskoutput tool results #983
• add taskoutput tool result formatter backed by nokogiri #960
• add task tool result formatter #963
• add agent tool result formatter #962
• add taskcreate tool result formatter #959
• add taskupdate tool result formatter #958
• add todowrite tool result formatter #957
• add skill tool result formatter #956
• add edit tool result formatter #955
• add write tool result formatter #954
• add grep tool result formatter #953 👈 (View in Graphite)
• add glob tool result formatter #952
• add read tool result formatter #951
• add bash tool result formatter #949
• Centralize tool result error handling in format with error_line helper #948
• main

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[juniper-shopify]

The overall structure is great — mirroring the GLOB formatter's partition + NOTE + ok_line pattern keeps the stack consistent and readable. The doc comment is thorough with clear examples.

One change needed: the match-detection heuristic is too loose and will misclassify some status messages as grep matches. Details in the inline comment.