Document common sandbox startup failures

[lpc0387]

1. Document common sandbox startup failures

Labels: good first issue, documentation, sandbox

Body:

New users may hit predictable sandbox setup problems: a busy port, missing editable install, a stale demo database, or confusion about mock mode versus --real. Add a short troubleshooting section to sandbox/README.md that explains these cases and points back to the existing safe defaults.

Suggested scope:

• Keep the section concise and beginner-friendly.
• Mention ./scripts/start_sandbox.sh --port 8877 for port conflicts.
• Mention that mock mode is the default and does not call real workers.
• Mention that generated databases and logs should not be committed.
• Do not add new runtime behavior.

Acceptance criteria:

• sandbox/README.md has a new troubleshooting section covering at least three common startup problems.
• The section preserves the repository safety boundary around mock data, runtime databases, and secrets.
• Existing quick-start commands remain unchanged unless they are being clarified.
• Verification notes include a Markdown review and, if shell snippets are changed, bash -n scripts/start_sandbox.sh.

Why it is newcomer-friendly:

This is a documentation-only task with a clear target file, concrete examples, and no need to understand the full collaboration engine internals.

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This issue was created from the v5.0 launch backlog in docs/launch/good-first-issues.md.