Fix sudo prompting and clean up provider plugins - #3

What changed

• Classify non-interactive sudo preflight results so permissive (never) mode prompts only when sudo explicitly requires a password.
• Run passwordless or cached sudo without a flyout and surface unavailable or denied sudo as normal command errors.
• Apply the behavior to agent bash calls and user bang commands.
• Remove obsolete built-in provider and OAuth helpers.
• Add the plugin-umans provider manifest.

Why

The previous sudo probe treated every non-zero result as a password requirement. Missing sudo, policy denials, and probe failures therefore opened a misleading password prompt even in permissive mode.

Impact

Users in never permission mode no longer receive unnecessary sudo prompts. Non-permissive modes retain their existing approval prompt behavior.

Validation

• cargo check --manifest-path core/Cargo.toml passes.
• git diff --check passes.
• Targeted unit tests are present, but the test build is currently blocked by config.rs tests that still reference the removed preset_has_oauth_creds helper included in this branch.