fix: remove hardcoded "main" default from default_project (#575)

When a user's config.json had projects with names other than "main" and
no explicit default_project key, the hardcoded field default of "main"
would not match any project. The model_post_init fixup logic existed but
was untested and only handled the stale-name case, not the None case.

Changes:
- Change default_project field default from "main" to None
- Use model_fields_set to distinguish "config omitted the key" (auto-resolve
  to first project) from "user explicitly set None" (preserve for discovery mode)
- Split model_post_init into two branches: auto-resolve when not explicitly
  provided, correct stale names when explicitly set but invalid
- Remove # pragma: no cover from now-tested branches
- Add 10 new tests covering valid defaults, stale defaults, empty string,
  single project, config file round-trips, and discovery mode preservation

Fixes #575

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Signed-off-by: phernandez <paul@basicmachines.co>

src/basic_memory/config.py

@@ -132,7 +132,7 @@ class BasicMemoryConfig(BaseSettings):
         description="Mapping of project names to their ProjectEntry configuration",
     )
     default_project: Optional[str] = Field(
-        default="main",
+        default=None,
         description="Name of the default project to use. When set, acts as fallback when no project parameter is specified. Set to null to disable automatic project resolution.",
     )
 
@@ -493,18 +493,25 @@ class BasicMemoryConfig(BaseSettings):
         if self.database_backend == DatabaseBackend.POSTGRES:  # pragma: no cover
             return  # pragma: no cover
 
-        # Ensure at least one project exists; if none exist then create main
-        if not self.projects:  # pragma: no cover
+        # Trigger: no projects configured (fresh install or empty config)
+        # Why: every config needs at least one project to be functional
+        # Outcome: creates "main" project using BASIC_MEMORY_HOME or ~/basic-memory
+        if not self.projects:
             self.projects["main"] = ProjectEntry(
                 path=str(Path(os.getenv("BASIC_MEMORY_HOME", Path.home() / "basic-memory")))
             )
 
-        # Ensure default project is valid (i.e. points to an existing project)
-        # None means "no default" — intentionally left unset
-        if (
-            self.default_project is not None and self.default_project not in self.projects
-        ):  # pragma: no cover
-            # Set default to first available project
+        # Trigger: default_project was not explicitly provided in the input data
+        #          (config file omitted the key, or BasicMemoryConfig() called with no args)
+        # Why: callers like get_project_config() expect a valid project name;
+        #      but explicit None (discovery mode) must be preserved
+        # Outcome: sets default_project to the first available project
+        if "default_project" not in self.model_fields_set:
+            self.default_project = next(iter(self.projects.keys()))
+        # Trigger: default_project was explicitly set but references a non-existent project
+        # Why: project may have been removed or renamed since config was saved
+        # Outcome: corrects to the first available project
+        elif self.default_project is not None and self.default_project not in self.projects:
             self.default_project = next(iter(self.projects.keys()))
 
     @property