Expand Changes/Archive trees to individual tasks: reveal + delete

[VeryComplexAndLongName]

Summary

• Each change node in the Changes and Archive tree views now also lists its tasks.md checklist items as children, alongside the existing artifacts.
• Selecting a task opens (or reveals, if already open) tasks.md with the cursor moved to that exact line, in both trees.
• "Delete Task" removes a single checklist line from an active change's tasks.md, with confirmation. Archived tasks never offer a delete action.
• deleteTaskLine() re-reads tasks.md fresh and re-verifies the target line's exact text before removing it, rather than trusting a potentially-stale line number captured when the tree was last refreshed — fails closed with a clear TaskListChangedError if the file changed underneath, instead of risking deletion of the wrong line.
• ArtifactTreeItem-mapping logic (previously duplicated identically in changes-tree.ts and archive-tree.ts) is now a single shared getChangeChildren() helper both providers call.

Test plan

• npm run verify green across all five workspaces (388 tests total), re-run after git add per this repo's English-policy-check gotcha.
• openspec change validate --strict tasks-tree-expand passes.
• Real smoke tests, not just unit tests: readTaskChecklist run against this very change's own live tasks.md (14 real tasks, correct line numbers/done state); deleteTaskLine run against a real temp file (correct deletion, and correct rejection of a stale re-delete attempt); a real @vscode/test-electron run (6/6 passing, confirming the extension activates cleanly with the new commands registered). Full detail in openspec/changes/tasks-tree-expand/smoke-test-notes.md.
• Actual tree-click / editor-jump interaction not driven live — no desktop-app UI automation tool available in this environment (same gap noted for the two prior VS-Code-UI-only changes this session); covered by 39 passing commands.test.ts/tree-provider tests instead, including a vscode-mock.ts extended specifically for TextEditorRevealType/Selection/real lineAt().text support.

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