Add CLI for Synapse public backend API

[droxer]

Motivation

• Provide a simple, local CLI to exercise the backend public integration API (/v1) for creating agent runs, posting follow-up messages, and retrieving run status/results to aid development, automation, and debugging.

Description

• Add a new CLI module at backend/public_api_cli.py implementing an async httpx client (PublicApiClient) that talks to the /v1 endpoints with bearer auth and JSON error handling.
• Implement top-level subcommands: run (create a new agent run), message (post a follow-up message to a conversation), status (fetch run status), and result (fetch run result with --format selection), and include UUID validation via uuid.UUID.
• Provide CLI flags and env var fallbacks for configuration: --base-url (or SYNAPSE_BASE_URL), --api-key (or SYNAPSE_API_KEY), and --timeout.
• Expose the CLI as a console script synapse-cli by adding [project.scripts] synapse-cli = "public_api_cli:main" to backend/pyproject.toml.

Testing

• Ran formatter: python -m ruff format public_api_cli.py, which reformatted the file successfully.
• Ran linter/check: python -m ruff check public_api_cli.py, and the check passed.
• No automated pytest test suite was executed as part of this change.

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Codex Task