refactor(tui): add #[non_exhaustive] to all pub enums missed in #4545

[bug-ops]

Description

10 pub enums in `zeph-tui` were missed by the #4545 non-exhaustive sweep. All are public API extension points and will gain new variants as TUI features evolve.

Affected enums

File	Enum
`widgets/diff.rs`	`DiffLineKind`
`widgets/subagents.rs`	`AgentManagerState`
`widgets/tool_view.rs`	`ToolKind`, `ToolStatus`
`command.rs`	`TuiCommand`
`event.rs`	`AppEvent`, `AgentEvent`
`error.rs`	`TuiError`
`types.rs`	`InputMode`, `MessageRole`

Expected Behavior

All extensible pub enums have `#[non_exhaustive]`.

Actual Behavior

None of the above enums have `#[non_exhaustive]`.

Fix

Add `#[non_exhaustive]` above each enum definition. No logic changes required.

Environment

• Commit: HEAD main
• Crate: zeph-tui