chore(scripts): handle simple to_additive deprecations

[Fieldnote-Echo]

Closes #38550.

This updates scripts/add_deprecations.sh so that, when a renamed declaration is near an existing @[to_additive] attribute, the script also emits the additive deprecation alias in the common default-name case.

For example, foo_mul -> bar_mul now produces:

@[deprecated (since := "...")]
alias foo_add := bar_add

@[to_additive existing, deprecated (since := "...")]
alias foo_mul := bar_mul

This is intentionally conservative and text-based, matching the existing script. It handles common name translations such as mul/add, prod/sum, one/zero, inv/neg, and div/sub, and skips cases that look like they need less common to_additive name generation.

Tested with a local six-case fixture and a scratch Lean file confirming that the generated alias form compiles and produces deprecation warnings for both names.

I opened this as draft because I still count as a new contributor under the LLM-assisted PR policy. I have one merged mathlib PR (#36443), and this is a one-file script patch that I reviewed/tested manually. I'll leave it as draft unless a maintainer gives it the green light to flip ready.

I used Claude assistance while drafting and testing the patch. I reviewed the final diff manually and tested the generated Lean form locally.

[github-actions]

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[github-actions]

PR summary 35aa1b8156

Import changes for modified files

No significant changes to the import graph

Import changes for all files

Files	Import difference

---

Declarations diff

+ foo_add
+ foo_mul

You can run this locally as follows

## from your `mathlib4` directory:
git clone https://github.com/leanprover-community/mathlib-ci.git ../mathlib-ci

## summary with just the declaration names:
../mathlib-ci/scripts/pr_summary/declarations_diff.sh <optional_commit>

## more verbose report:
../mathlib-ci/scripts/pr_summary/declarations_diff.sh long <optional_commit>

The doc-module for scripts/pr_summary/declarations_diff.sh in the mathlib-ci repository contains some details about this script.

---

No changes to technical debt.

This script lives in the mathlib-ci repository. To run it locally, from your mathlib4 directory:

git clone https://github.com/leanprover-community/mathlib-ci.git ../mathlib-ci
../mathlib-ci/scripts/reporting/technical-debt-metrics.sh pr_summary

• The relative value is the weighted sum of the differences with weight given by the inverse of the current value of the statistic.
• The absolute value is the relative value divided by the total sum of the inverses of the current values (i.e. the weighted average of the differences).