feat: v0.5.0 — nested Joplin notebooks, date-prefixed note titles, flat year folders

Joplin notebooks now use a two-level hierarchy: AI-ChatGPT / <project> and
AI-Claude / <project> instead of a single flat title. Note titles are prefixed
with the conversation created_at date (YYYY-MM-DD). Export folders collapse
provider/project/year into a single provider/project.year directory.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/utils.py

@@ -50,7 +50,7 @@ def build_export_path(
         created_at: ISO8601 creation timestamp (used for year folder).
         filename: Already-generated filename from generate_filename().
         structure: OUTPUT_STRUCTURE value. One of:
-            "provider/project/year" (default)
+            "provider/project/year" (default) — project and year combined, e.g. no-project.2025/
             "provider/project"
             "provider/year"
 
@@ -64,14 +64,14 @@ def build_export_path(
     parts: list[str] = [provider]
 
     if structure == "provider/project/year":
-        parts += [project_slug, year]
+        parts += [f"{project_slug}.{year}"]
     elif structure == "provider/project":
         parts += [project_slug]
     elif structure == "provider/year":
         parts += [year]
     else:
         # Unknown structure — fall back to default
-        parts += [project_slug, year]
+        parts += [f"{project_slug}.{year}"]
 
     return base_dir.joinpath(*parts) / filename