[AIR #1074] vscode: align backlog issue-preview placement with count-then-pick (remove focus side-effect)

[amrmelsayed]

Summary

Aligns the backlog issue-preview placement with the count-then-pick pattern the builder/shell terminals already use (Bugfix #804), removing the focus side-effect that view-issue.ts previously relied on.

The old code (view-issue.ts:173-174) did a two-step dance:

await vscode.commands.executeCommand('workbench.action.focusFirstEditorGroup');
await vscode.commands.executeCommand('markdown.showPreviewToSide', uri);

It worked only because focusing group 1 first pinned Beside to group 2. That stole focus from wherever the user was sitting and, if any caller skipped the focus step, chained previews into groups 3/4/5.

Key decision: corrected API path

The issue proposed markdown.showPreview(uri, viewColumn, preserveFocus). I verified against the bundled VS Code 1.124.2 markdown extension that this command does not accept an explicit viewColumn and has no preserveFocus parameter — ShowPreviewCommand.execute(mainUri, allUris, {locked}) hardcodes sideBySide: false and always opens in the active editor group. I surfaced this to the architect rather than coding against the wrong signature.

The architect directed the corrected approach: open the built-in preview's custom-editor viewType via vscode.openWith, which does accept an explicit ViewColumn + TextDocumentShowOptions:

const viewColumn = pickIssuePreviewColumn(vscode.window.tabGroups.all.length);
await vscode.commands.executeCommand(
  'vscode.openWith', uri, 'vscode.markdown.preview.editor', { viewColumn, preserveFocus: true },
);

• vscode.markdown.preview.editor is VS Code's built-in markdown preview (verified in the markdown-language-features manifest: priority option, selector *.md — our codev-issue:<id>.md URI matches). It is distinct from Codev's own codev.markdownPreview artifact canvas, so this stays on the built-in renderer and does not overlap with artifact-canvas: revisit VSCode issue-preview migration once typography tokens (#1053) and comment system v2 (#1055) land (decision record) #1068.
• vscode.openWith precedent already exists in the codebase: extension.ts:804, commands/view-artifact.ts:127.

Changes

• commands/view-issue.ts: drop focusFirstEditorGroup; add the pure helper pickIssuePreviewColumn(groupCount) (group 2 when >= 2, else group 1 — if/else, mirroring terminal-manager.ts); call vscode.openWith with an explicit column + preserveFocus: true.
• src/__tests__/view-issue-column.test.ts: unit test for the column decision logic (mocked tabGroups.all.length → 1/2/3/5/0), following the diff-nav.test.ts minimal-vscode-mock pattern.
• Cache / refresh / close-cleanup (IssueContentProvider, OverviewCache.onDidChange, onDidCloseTextDocument) untouched.

Acceptance criteria

• No more workbench.action.focusFirstEditorGroup.
• Explicit viewColumn derived from vscode.window.tabGroups.all.length.
• Group 2 when a second group exists; group 1 otherwise.
• Focus preserved on the click origin (preserveFocus: true).
• No group-chaining / Beside fragility (explicit column, not Beside).
• Unit test for the viewColumn decision logic.
• No regression to cache / refresh / close-cleanup.

Note: the criterion as literally worded named markdown.showPreview, but that command cannot take an explicit column (verified, see above). The architect approved vscode.openWith + the built-in preview viewType as the correct way to satisfy the intent of every criterion.

Test plan

• pnpm check-types — clean
• eslint src/commands/view-issue.ts src/__tests__/view-issue-column.test.ts — clean
• pnpm test:unit (full vitest run) — 35 files / 446 tests pass (incl. 4 new)
• Manual (recommended at review): click a backlog row with a 2nd editor group open → preview lands in group 2, focus stays on the sidebar; repeat-click → reuses the same group (no 3/4/5 chaining); with a single group → preview lands in group 1.

Closes #1074.