Create agent plan content directory for backups

[loglapa]

What changed

Create ${GITLAB_SHARED_DIR}/agent_plan_content during data-directory initialization with the expected git:git ownership and 0700 permissions.

Why

GitLab backups from 19.1 can fail with ENOENT when the agent plan content directory has not been created yet. The directory is a configured backup path, but docker-gitlab does not currently initialize it.

This implements the working directory-creation workaround reported in #3269 as an idempotent container lifecycle step. The related upstream GitLab bug remains open: https://gitlab.com/gitlab-org/gitlab/-/work_items/604413

Impact

Fresh and upgraded installations get the expected directory before a backup runs, avoiding a backup failure without requiring a manual workaround.

Validation

• bash -n assets/runtime/functions
• git diff --check
• Executed the install -d command twice in a sameersbn GitLab container and verified directory type, git:git ownership, and mode 0700

Fixes #3269