docs(extensions): fix private catalog FAQ command

[WOLIKIMCHENG]

Summary

• Replace the obsolete private-catalog FAQ command with specify extension catalog add.
• Document the required project context and catalog name.
• Remove the stale Phase 4 implementation note while retaining the safe discovery-only default.

Validation

• .venv/bin/specify extension catalog add --help
• git diff --check origin/main
• Verified command and default-policy parity against the current implementation and extension references.

[Copilot]

🟢 Approval recommended

The documentation matches the implemented command syntax and discovery-only default.

Pull request overview

Updates the private-extension FAQ to match the current project-scoped catalog CLI.

Changes:

• Replaces the obsolete command.
• Documents the required catalog name and project context.
• Removes the outdated implementation note.

File summaries

File	Description
extensions/EXTENSION-PUBLISHING-GUIDE.md	Corrects private catalog setup guidance.

Review details

• Files reviewed: 1/1 changed files
• Comments generated: 0
• Review effort level: Balanced

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