enhance(pull)!: add per-repository default squash merge commit message (#39816)

Adds a repository setting for the default squash merge commit message
body with GitHub's options `PR_BODY`, `COMMIT_MESSAGES` and `BLANK`,
also settable via `squash_merge_commit_message` in the API. The commit
title stays the pull request title.

1. Breaking: API squash merges without `merge_message_field` now get the
same default message as the web merge box instead of a title-only
commit, which affects clients sending only `Do` like Renovate. An empty
value keeps title-only.
2. Breaking: `[repository.pull-request] DEFAULT_SQUASH_MERGE_MESSAGE`
replaces `POPULATE_SQUASH_COMMENT_WITH_COMMIT_MESSAGES`, which still
works as a deprecated fallback. Repositories store the default when
created or next saved.
3. Deprecated app.ini keys now migrate through one table instead of
per-loader fallbacks, which also fixes the ignored `[mailer] CERT_FILE`
and `KEY_FILE` fallbacks. Deprecation warnings no longer claim a removal
version.

<img width="1891" height="571" alt="grafik"
src="https://github.com/user-attachments/assets/0fdcde13-1f11-4156-b598-d80eb0d04cbe"
/>

---------

Co-authored-by: silverwind <me@silverwind.io>

modules/structs/repo.go

@@ -120,6 +120,7 @@ type Repository struct {
 	DefaultDeleteBranchAfterMerge bool             `json:"default_delete_branch_after_merge"`
 	DefaultMergeStyle             string           `json:"default_merge_style"`
 	DefaultUpdateStyle            string           `json:"default_update_style"`
+	SquashMergeCommitMessage      string           `json:"squash_merge_commit_message"`
 	DefaultAllowMaintainerEdit    bool             `json:"default_allow_maintainer_edit"`
 	AvatarURL                     string           `json:"avatar_url"`
 	Internal                      bool             `json:"internal"`
@@ -236,6 +237,8 @@ type EditRepoOption struct {
 	DefaultMergeStyle *string `json:"default_merge_style,omitempty"`
 	// set to an update style to be used by this repository: "merge" or "rebase".
 	DefaultUpdateStyle *string `json:"default_update_style,omitempty"`
+	// set the default squash merge commit message body: "PR_BODY", "COMMIT_MESSAGES", or "BLANK".
+	SquashMergeCommitMessage *string `json:"squash_merge_commit_message,omitempty"`
 	// set to `true` to allow edits from maintainers by default
 	DefaultAllowMaintainerEdit *bool `json:"default_allow_maintainer_edit,omitempty"`
 	// set to `true` to archive this repository.