feat: display lines numbers in diff

[sudo-tee]

No description provided.

[Copilot]

Pull request overview

Adds diff gutter line numbers to the UI diff formatter output (rendered via extmarks), and updates test fixtures accordingly.

Changes:

• Update formatter.format_diff to compute per-line old/new line numbers from hunk headers and render a gutter via overlay extmarks.
• Add new diff gutter highlight groups for neutral/add/delete gutters.
• Update unit tests and expected JSON fixtures to reflect the new gutter/extmark output.

Reviewed changes

Copilot reviewed 9 out of 10 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
lua/opencode/ui/formatter.lua	Parses diff hunks to compute line numbers and renders a line-number gutter via extmarks.
lua/opencode/ui/highlight.lua	Adds highlight groups for the new diff gutter styling.
tests/unit/formatter_spec.lua	Adds a unit test validating diff line numbers are rendered as extmarks.
tests/data/shifting-and-multiple-perms.expected.json	Updates expected extmarks/lines output to include gutter changes.
tests/data/redo-once.expected.json	Updates expected output for diff gutter and resulting display/action ranges.
tests/data/redo-all.expected.json	Updates expected output structure/ordering to match new rendered lines and gutter extmarks.
tests/data/markdown-codefence.expected.json	Updates expected extmarks/lines to include gutter rendering in diffs.
tests/data/diff.expected.json	Updates expected extmarks/lines for diff gutter behavior.
tests/data/apply-patch.expected.json	Expands and reorders expected JSON output to include gutter extmarks and formatted lines.

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