Make plan interactive mode explicit via --interactive

[MicheleBonus]

Motivation

• Prevent accidental blocking prompts when invoking ambermeta plan with no mode selected by making interactive prompting opt-in.
• Clarify CLI UX and docs so users explicitly choose manifest, recursive discovery, or interactive mode.

Description

• Require one of --manifest, --recursive, or the new --interactive flag in _plan_command, printing actionable help and returning non-zero (2) when none are provided (file: ambermeta/cli.py).
• Add a --interactive option to the plan parser and update plan help/usage examples to show explicit interactive invocation (file: ambermeta/cli.py).
• Update user-facing documentation and examples to avoid implying interactive is the default (README.md, docs/cli.md).
• Add tests/test_cli_plan.py with tests that assert the new default behavior, --interactive parsing, and opt-in interactive execution.

Testing

• Ran pytest -q tests/test_cli_plan.py tests/test_cli_init.py tests/test_cli_protocol_output.py, and all tests passed (10 passed).
• Unit tests added: test_plan_requires_explicit_mode, test_plan_parser_accepts_interactive_flag, and test_plan_interactive_mode_runs_only_with_opt_in, which all succeeded.

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Codex Task