Split PluginBase: extract DiscoverablePlugin for CLI plugins

[renaudcepre]

Context

PluginBase mixes two responsibilities:

1. Discovery/CLI — name, description, add_cli_options(), activate()
2. Lifecycle hooks — setup(), on_test_pass(), on_session_end(), etc.

Internally-wired plugins (HistoryPlugin, EvalHistoryPlugin, EvalResultsWriter) only need lifecycle hooks but are forced to override activate() with return None to prevent auto-discovery. This is a code smell.

Proposal

PluginBase              → lifecycle hooks only (setup, on_*)
    ↑
DiscoverablePlugin      → adds name, description, add_cli_options, activate

• Internal plugins inherit PluginBase
• CLI-discoverable plugins inherit DiscoverablePlugin
• No more defensive return None overrides

Additional motivation: circular import

core/session.py imports evals/history, evals/results_writer, and evals/wrapper to wire eval plugins manually. This creates a dependency inversion — core depends on evals, instead of evals plugging into core. This forces a __getattr__ lazy import hack in evals/__init__.py to break the cycle.

If eval plugins were auto-discovered (via DiscoverablePlugin.activate()), session wouldn't need to import them directly, eliminating the circular dependency entirely.

Files affected

• protest/plugin.py — split the class
• All plugins that currently override activate() -> None — simplify to PluginBase
• Auto-discovery in session — only scan DiscoverablePlugin subclasses
• protest/core/session.py — remove manual eval plugin wiring (_wire_eval_support)
• protest/evals/__init__.py — remove __getattr__ hack once cycle is broken