Granular tool permission controls — allow-list specific tools

## Description:

Currently, Tool permissions only offers two options: "Approve all" or "Ask every time". There is no way to allow specific tools automatically while still requiring approval for others.

## Requested feature:

Add per-tool permission configuration in Settings → Sessions → Tool permissions, allowing users to define an allow-list of tools that run without confirmation (e.g. grep, glob, view) while all other tools still require explicit approval.

## Use case:

Read-only tools like grep/ripgrep pose no risk to the codebase and interrupting the agent flow to approve every search is disruptive. Write/execute tools (bash, file edit, git push) should still require confirmation. A middle ground between "trust everything" and "approve everything" would significantly improve agent UX without sacrificing safety on destructive operations.

## Proposed UI:

In the Tool permissions dropdown, add a third option: "Custom" — opens a list of available tools with individual allow/ask toggles per tool.

从 Settings → Sessions → Tool permissions 开始，追踪现有的“Approve all”和“Ask every time”选项是如何表示和应用的。完成标准是“Custom”选项能够提供按工具设置的 allow/ask 切换，其中列入 allow-list 的工具跳过确认，而所有其他工具仍然提示确认，包括只读以及 write/execute 示例。

由索引模型根据 Issue 内容生成。