fix(vscode): remove duplicate command contribution

[Totoro-qaq]

Description

Fixes #13158.

The VS Code manifest contributed continue.focusContinueInput twice, giving it 38 command entries but only 37 unique command IDs. This removes the duplicate contribution, keeps the canonical Focus Continue Chat title, and adds a manifest regression test that rejects duplicate command identifiers.

This prevents VS Code from receiving two registrations for the same Continue command during extension startup, including in WSL extension hosts.

AI Code Review

• Team members only: AI review runs automatically when PR is opened or marked ready for review
• Team members can also trigger a review by commenting @continue-review

Checklist

• I've read the contributing guide
• The relevant docs, if any, have been updated or created (not applicable; no user-facing workflow or configuration changed)
• The relevant tests, if any, have been updated or created

Screen recording or screenshot

Not applicable — this fixes extension manifest metadata and does not change the UI.

Tests

• npm test (7 files, 87 tests passed)
• npm run tsc:check
• npm run lint (0 errors)
• ../../node_modules/.bin/prettier --check package.json src/packageManifest.vitest.ts
• git diff --check

[Totoro-qaq]

I have read the CLA Document and I hereby sign the CLA

[github-actions]

All contributors have signed the CLA ✍️ ✅
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[Totoro-qaq]

CI note: the code-related checks are green (lint, Prettier, VS Code checks, core/gui/binary/package checks, and all Node test matrices). The remaining failures are runner/network infrastructure:

• Windows VSIX: sharp prebuild download reset (ECONNRESET), then its fallback node-gyp could not recognize the runner's Visual Studio 18 install.
• JetBrains: the FFmpeg setup action downloaded a non-xz response (File format not recognized).
• macOS VSIX completed its build and artifact uploads, then was cancelled by fail-fast.

A maintainer rerun should clear these; GitHub does not allow fork contributors to rerun the upstream jobs.