agent: lumotia — Phase A.3 remove dead migration_sentinel method + fix architecture-map claim

Phase A.3 finding: AppPaths::migration_sentinel was added with intent
during the rebrand-architecture phase but never wired to any caller.
Exhaustive grep across crates/ src-tauri/ src/ docs/ surfaces:

  - 1 definition (paths.rs)
  - 1 architecture-map description that ASSERTS the method is in use
  - 0 production callers
  - 0 test references

Both boot-time migrations (migrate_legacy_data_dir +
migrate_tauri_app_data_dir_with_paths) are idempotent by construction:
each re-probes the legacy path via Path::exists() on every boot and
short-circuits on the steady state. A sentinel file would optimise the
probe but is not required for correctness; one syscall per legacy
candidate at startup is negligible.

Per the atomiser principle of removing dead surface area rather than
keeping stale promises:
  - Delete AppPaths::migration_sentinel entirely
  - Update docs/architecture-map/.../core-paths.md to describe the actual
    idempotency model (re-probing) rather than the sentinel pattern that
    was never implemented
  - Steer future migrations toward storage/src/migrations.rs schema_version
    (transactional, survives backup/restore) rather than reintroducing
    filesystem sentinels

Verification:
- cargo test -p lumotia-core paths::: 17/17 (no test relied on the method)
- cargo clippy -p lumotia-core --all-targets -- -D warnings: clean

Phase A.4 (stray-magnotia string scan): clean. Every magnotia reference
in the tree is legitimate — migration source paths, documentation, or
test fixtures. The rebrand cascade was thorough.

crates/core/src/paths.rs

@@ -49,10 +49,6 @@ impl AppPaths {
     pub fn llm_models_dir(&self) -> PathBuf {
         self.models_dir().join("llm")
     }
-
-    pub fn migration_sentinel(&self, name: &str) -> PathBuf {
-        self.app_data_dir.join(format!(".{name}.sentinel"))
-    }
 }
 
 pub fn app_paths() -> AppPaths {