add opencode skill for testgrid

[xavpaice]

What this PR does / why we need it:

Add a skill for Opencode to read Testgrid results and start to analyze them

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Steps to reproduce

Does this PR introduce a user-facing change?

NONE

Does this PR require documentation?

NONE

[greptile-apps]

Comments Outside Diff (1)

1. .gitignore, line 33-34 (link)

   .opencode directory is still gitignored despite committed skill files

   The root .gitignore retains the .opencode entry (in the # ai/editor metadata block), but this PR adds tracked files under .opencode/. Because they were committed via git add -f, existing contributors can work with them fine, but any contributor who tries to add new skill files (e.g., a second skill) will need to use git add -f to bypass the ignore rule — which is non-obvious and easy to miss. The .opencode line should be removed from .gitignore (or narrowed to only ignore transient artifacts like node_modules) so the directory is treated as a normal part of the source tree.

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