feat: open tabs from origin panel group

[leynier]

What changed

• Adds optional sourceKey to file, editor, markdown, merman preview, git diff, git commit diff, and pull request diff open paths and resolves the opening group via _groupForOpening.
• Scopes preview-tab replacement to the resolved target group, preferring the active preview tab in that group.
• Threads the origin key from context-sidebar tools for explorer, search, source control, and pull requests, from single-tab surfaces, from quick open's focused panel key, from terminal composer tabs, from editor to diff and diff to editor actions, and from the pull requests panel.
• Keeps queued opens pinned to their captured origin group when focus changes, while already-open files are still selected in their existing location.
• Updates readme.md file explorer behavior and adds workbench_controller_origin_panel_test_cases.dart coverage, with test fakes updated for the new signatures.

Why

• Files and previews opened from a tab or tool now stay in its panel and split group, including explorer in either the center or the collapsible right panel, instead of following the currently focused group.

[pullfrog]

✅ No new issues found.

Reviewed changes Origin sourceKey pins file, preview, and diff opens to the originating panel group instead of the focused one.

• Origin group resolution. _groupForOpening maps a tool or tab key onto the main or right layout, and _openReplaceableTab snapshots that group before the open queue so a later focus change cannot steal the tab.
• Scoped preview replacement. _previewTabIdInGroup only replaces a preview in the resolved group, preferring that group's active preview.
• Call-site threading. Explorer, search, source control, pull requests, single-tab surfaces, Quick Open, terminal composer, editor-to-diff, and diff-to-editor pass the origin key.
• Tests. New origin-panel cases cover both trees, in-flight opens, split-group placement, and keeping an already-open file where it already lives.

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