## Why
Executor hooks depended on V1 discovery snapshots, so plugins discovered through V2 could not run their cleanup hooks.
## What changed
Prepare executor hook sources during step setup from either V1 or V2 discovery and capture them in `StepContext` for stop, interrupt, and memory consolidation hooks. Restrict V2 sources to selected plugin installations while preserving plugin enablement, hook admission, MCP routing, and app tool policy checks.
## Testing
Add unit coverage for V2 hook admission, environment routing, disabled plugins, and failed discovery. Add integration coverage showing that `Stop` and `Interrupt` cleanup hooks run with V2 discovery alone and remain silent when the plugin is disabled.
GitOrigin-RevId: a4c0e9c56bdebcdf22cea63f02f8fb4c5eed358e
