feat: Changes to generate sdk changelog in releases.md and pr description

[kanwardeep007]

Caution

Dont use local packages in go.mod
Revert commit 184fc89

These changes enable the sdk action to use the new SDK changelog in the following places

• Release notes for each github release
• PR description when speakeasy updates sdks
• As commits messages create by speakeasy

This feature is currently behind a environment variable ENABLE_SDK_CHANGELOG

• setting this to true enabled the new changelog

Approach

We can take one of 2 types of release/pr/commit message. We decide on the basis of environment flag SDK_CHANGELOG_JULY_2025.
DO NOTE: for terraform we do not use the new format

Currently this SDK changelog is automatically enabled for all repos which have the word speakeasy in the URL. This is to check and tweak the changelog in for the repos at the following URLs

• https://github.com/speakeasy-sdks/
• https://github.com/speakeasy-sdks-dev/

Impact analysis for different permutations of cli and sdk-action

CLI	sdk-action	commit message	pr message	release notes
CLI	sdk-action	commit message	pr message	release notes
old cli	old action	works	works	works
old cli	new action	commitmessage - commitreport would be empty because its not written by old cli. So mergedVersionReport.GetCommit MarkdownSection() will be empty	PR details would remain unchanged bec cli is old	release notes would be unchanged because we dont write release notes in older gen.lock files. So sdk-action wont find anything there
new cli	old action (in the action we try and download the latest cli so this row is more likely than row 2)	remain same	We add new version reports only when env is set. And because env var wont be set for all of our clients the pr message will remain the same	remain same
new cli	new action	should work	should work	should work

Related PRs

• feat: Save release notes in gen.lock file  sdk-gen-config#86
• https://github.com/speakeasy-api/openapi-generation/pull/2895
• feat: sdk changelog speakeasy#1495

Notes on the PR

• Github is showing large diffs in internal/git/git.go but it would be easier to see the old file and the new file side by side without the diffs.

The changes in internal/git/git.go are as follows

• refactored code into methods
• Introduced a new method to generate new pr description (hid it behind an environment variable )