feat: add {{projectFolder}} folder template variable

[carypruitt]

What

Adds two folder template variables, {{projectFolder}} and {{projectFolders}}, usable anywhere folder-path templates are processed (Default tasks folder, folder for inline-created tasks, etc.).

{{projectFolder}} resolves to the folder containing the first linked project note, i.e. the project's file path with the note's own basename removed. {{projectFolders}} does the same for every linked project, joined with /.

Why

There is currently no way to store a task in the same folder as its project note. The two existing project path tokens each miss:

• {{project}} returns the note's basename with no folder. It only lands a task beside its project note when the note sits at the vault root in a folder matching its own name.
• {{projectFilePath}} returns the full path including the note basename, which nests an extra folder: a task for [[Areas/EFC/EFC]] lands in Areas/EFC/EFC/ rather than Areas/EFC/.

{{projectFolder}} covers the general case.

Project link	{{projectFilePath}}	{{projectFolder}}
[[Areas/EFC/EFC]]	Areas/EFC/EFC	Areas/EFC
[[EFC]] (root)	EFC	`` (empty)

With {{projectFolder}} as the Default tasks folder, a task added to project [[Areas/EFC/EFC]] is created in Areas/EFC/, next to the note.

Implementation

• New getProjectFolder helper in src/utils/folderTemplateProcessor.ts, built on the existing getProjectFilePath so wikilink/markdown-link parsing and path sanitization are shared. It drops the final path segment and returns an empty string when the project note has no containing folder.
• Two .replace calls added to processFolderTemplate, following the existing {{projectFilePath}} / {{projectFilePaths}} pattern.
• Docstring, docs/features/template-variables.md, and docs/settings/task-defaults.md updated.

Tests

Added a {{projectFolder}} and {{projectFolders}} block to tests/unit/utils/folderTemplateProcessor.test.ts covering nested notes, top-level notes (empty result), the store-beside-project-note case, multi-project joining with empties dropped, and the no-projects case. All pass.

Note: one unrelated pre-existing test in this file (issue #816 specific use case) is timezone-sensitive and fails under non-UTC local time (off by one day); it passes under TZ=UTC and is untouched by this change.