The type guards for `file_edit_replace` and `file_edit_insert` tools were checking for the presence of `lease` in args, which is no longer part of the tool API after PR #136.

Removes the `"lease" in args` checks from:
- `isFileEditReplaceTool()` type guard
- `isFileEditInsertTool()` type guard

This allows the UI components to properly recognize and display these tool calls without the lease parameter.

## Problem

After removing the lease concept from file operations in #136, the type
guards in `ToolMessage.tsx` were still checking for the presence of
`lease` in the tool arguments. This exposed a deeper architectural
issue: **hand-written type guards can drift from type definitions**.

## Root Cause

Type guards used manual property checks that TypeScript can't verify
match the actual types:

```typescript
// ❌ Problem: Can get out of sync with type definition
function isFileEditReplaceTool(toolName: string, args: unknown): args is FileEditReplaceToolArgs {
  return (
    toolName === "file_edit_replace" &&
    typeof args === "object" &&
    args !== null &&
    "file_path" in args &&
    "edits" in args &&
    "lease" in args  // ← TypeScript doesn't verify this matches FileEditReplaceToolArgs!
  );
}
```

## Solution: Schema-Driven Type Guards

Use the existing Zod schemas from `toolDefinitions.ts` as the single
source of truth:

```typescript
// ✅ Solution: Uses schema validation - always in sync
import { TOOL_DEFINITIONS } from "@/utils/tools/toolDefinitions";

function isFileEditReplaceTool(toolName: string, args: unknown): args is FileEditReplaceToolArgs {
  if (toolName !== "file_edit_replace") return false;
  return TOOL_DEFINITIONS.file_edit_replace.schema.safeParse(args).success;
}
```

## Benefits

1. **Single source of truth**: Zod schemas define both runtime
validation and TypeScript types
2. **Automatic sync**: Schema changes automatically propagate to type
guards
3. **More robust**: Validates full structure, not just key presence
4. **Future-proof**: Prevents entire category of drift bugs

## Changes

- Import `TOOL_DEFINITIONS` from `toolDefinitions.ts`
- Replace all manual type guards with schema-based validation
- Apply to all tools: `bash`, `file_read`, `file_edit_replace`,
`file_edit_insert`, `propose_plan`
- Net result: -33 lines of manual validation code, +13 lines of
schema-driven validation

## Related

- Depends on #136 (Remove lease concept from file operations)

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