docs(core,cli): document implicit contracts flagged in code review

- import_lastpass.rs: note that password and extra are intentionally
  not trimmed (leading/trailing whitespace is significant for both).
- cmd_import_lastpass: document the coupling between the
  ImportWarning message strings and the CLI summary's "skipped"
  filter — partial-import warnings (TOTP/URL) must not contain
  the word "skipped".

Comment-only; no behavior change. Catches I1 and M5 from the
final code review without taking on the cross-cut WarningKind
enum refactor (deferred to a follow-up if it ever ships).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

crates/relicario-core/src/import_lastpass.rs

@@ -95,6 +95,8 @@ fn map_row(
 ) -> (Option<Item>, Option<ImportWarning>) {
     let url      = record.get(0).unwrap_or("").trim();
     let username = record.get(1).unwrap_or("").trim();
+    // password and extra are deliberately NOT trimmed: leading/trailing
+    // whitespace is significant inside passwords and free-form notes.
     let password = record.get(2).unwrap_or("");
     let totp_raw = record.get(3).unwrap_or("").trim();
     let extra    = record.get(4).unwrap_or("");