Builds on memosr's sink-level opt-in gate (#29249). Enabling a
non-bundled plugin now surfaces the privileged allow_tool_override
decision at `hermes plugins enable` time instead of leaving the
operator to discover the config key after a runtime rejection.
- `hermes plugins enable <name>` prompts for non-bundled plugins:
'Allow this plugin to replace built-in tools?' Default is deny
(blank Enter / non-interactive stdin / EOF all fail closed).
- --allow-tool-override / --no-allow-tool-override flags for
non-interactive and scripted use (and a future desktop checkbox).
- Bundled plugins are trusted: never prompted, no entry written.
- Writes plugins.entries.<key>.allow_tool_override, the same key the
sink gate reads (manifest.key == discovery key), so consent and
enforcement compose end to end.
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