Add configuration prompt when no issues found in completion

[Copilot]

When typing # to trigger issue completion and no issues are found, users previously saw an empty completion list with no guidance on how to configure their issue queries. This made the feature less discoverable and left users unsure how to set up issue completion.

Changes Made

Added a helpful configuration prompt that appears when no issues are found:

• New completion item: "Configure issue queries..." with detail text "No issues found. Set up queries to see relevant issues."
• Direct settings access: Selecting the prompt opens VS Code settings filtered to @ext:GitHub.vscode-pull-request-github githubIssues.queries
• No text insertion: The prompt doesn't insert any text, preserving the user's input

Behavior

Before:

# (typing # shows empty completion list)

After:

# (typing # shows configuration prompt when no issues found)
┌─────────────────────────────────────────────────┐
│ 📝 Configure issue queries...                  │
│    No issues found. Set up queries to see rel... │
└─────────────────────────────────────────────────┘

Implementation Details

• Uses existing workbench.action.openSettings command pattern found throughout the codebase
• Properly localized with vscode.l10n.t() for all user-facing strings
• Handles all edge cases: no queries configured, queries returning empty results, and issues filtered out by owner/repo
• Preserves existing behavior when issues are available
• Maintains markdown header detection and other existing completion logic

The prompt only appears when completionItems.size === 0, ensuring it doesn't interfere with normal issue completion when issues are available.

Fixes #7259.

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[Copilot]

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