fix(core): normalize file.status paths relative to instance dir

[shantur]

Issue for this PR

None

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

file.status returns incorrect/unstable file paths when x-opencode-directory points inside a repo (including git worktrees) because it runs path.relative(Instance.directory, x.path) on git-provided paths that are already relative, causing bogus ../... paths and inconsistent results depending on server cwd and request directory.

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How did you verify your code works?

Manual API verification and tested with NeuralNomadsAI/CodeNomad#180

Screenshots / recordings

Pre Change

[
    {
        "path": "../../../banner.py",
        "added": 0,
        "removed": 1,
        "status": "modified"
    },
    {
        "path": "../../../tasmotizer.py",
        "added": 150,
        "removed": 2,
        "status": "modified"
    }
]

Post Change

[
    {
        "path": "banner.py",
        "added": 0,
        "removed": 1,
        "status": "modified"
    },
    {
        "path": "tasmotizer.py",
        "added": 150,
        "removed": 2,
        "status": "modified"
    }
]

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

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[github-actions]

This PR doesn't fully meet our contributing guidelines and PR template.

What needs to be fixed:

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[github-actions]

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