[FEATURE] VS Code: Add setting to disable plan opening as separate markdown tab

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

Since the recent addition of "full markdown document view for plans," Plan mode in the VS Code extension now opens the plan as a separate editor tab. There is no setting to keep the plan inline within the Claude Code panel.

This is a significant regression for users running multiple Claude Code sessions in parallel. When 4–5 sessions are active simultaneously (via separate tabs or windows), each plan opens as a near-identical markdown tab with no clear visual tie to its originating session. The result:

• Plans become impossible to attribute. Multiple plan tabs open at once with similar-looking content. There's no indication which Claude session produced which plan.
• Context switching overhead. Instead of reading the plan in the same panel where the conversation is happening, focus is pulled to a separate tab — breaking flow and requiring manual navigation back.
• Tab clutter compounds with parallelism. With N sessions × plan tabs × file edit tabs, the editor tab bar becomes unusable.

The inline comment feature that motivated this change is nice in theory, but the tradeoff is not worth it for parallel workflows — and there's no way to opt out.

Proposed Solution

Add a VS Code setting (e.g. claudeCode.openPlanInEditor: true/false) that controls whether plans open as a separate markdown document or remain inline in the Claude Code panel.

• Default: true (current behavior preserved for users who want inline comments on plans)
• When false: Plans render inside the Claude Code chat panel as before, without opening a separate editor tab.

Alternatives Considered

• Switching to the terminal-based CLI ("Use Terminal" setting) avoids the issue but sacrifices the graphical UI features of the extension.
• Manually closing plan tabs after each generation — not viable at scale with multiple sessions.

Additional Context

Related: #25567 (request to disable auto-opening files on Write/Edit) — same category of forced tab-opening with no opt-out. The underlying principle is the same: let users control what opens in their editor area, especially in multi-session workflows.