[A06] Tool Runtime refactoring discussion and implementation plan

## Purpose

This issue is the discussion and coordination entry point for A06 from [monthly cleanup issue #4726](https://github.com/apache/maka/issues/4726).

A06 covers:

- `packages/runtime/src/tool-runtime.ts`
- `packages/runtime/src/computer-use-tools.ts`

The current checkout contains 7,063 lines across these two files. The goal is to reduce structural duplication and clarify ownership through several small, reviewable refactoring slices.

This work does not add product capabilities. Each implementation slice must preserve public APIs, provider behavior, event formats, and existing runtime semantics.

The overall analysis and proposed target architecture are recorded in the second comment. Implementation work is tracked through linked child issues.

## Child issues

- [#4908 — Extract the existing tool-call snapshot and centralize argument projections](https://github.com/apache/maka/issues/4908)

Additional child issues will be added after each boundary is reviewed.

Read the second comment for the proposed target architecture, then inspect packages/runtime/src/tool-runtime.ts and packages/runtime/src/computer-use-tools.ts. Start with child issue #4908 to understand the first implementation slice; done means each reviewed slice preserves public APIs, provider behavior, event formats, and existing runtime semantics.

Written by the indexing model from the issue text.