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-cli/src/main.rs

@@ -1618,6 +1618,11 @@ fn cmd_import_lastpass(csv_path: PathBuf) -> Result<()> {
     for w in &warnings {
         print_warning(w);
     }
+    // Counts only true skips, not partial imports. Coupled by convention to
+    // the parser's warning message strings: skip messages end in "— skipped",
+    // partial-import messages say "imported without TOTP" / "imported without URL".
+    // If a future warning uses the word "skipped" in any other sense, this filter
+    // will need to switch to an enum tag (see ImportWarning::message).
     eprintln!(
         "Imported {}, skipped {} (see warnings above)",
         total,