fix: print repository error prefix once; reject blank hsearch queries

[sysid]

Summary

Follow-up to #80, which was merged before these two commits were pushed.

Commit	Problem	Fix
fix(errors): print the repository error prefix once	Errors read Repository error: Repository error: … — RepositoryError::Other repeated the prefix its DomainError wrapper already adds	Other displays the bare message, like all other Other variants
fix(hsearch): reject blank queries with a validation error	Empty/whitespace query reached FTS5: fts5: syntax error near ""	Validation error before any search runs, pointing to search for tag-only browsing

Before / after:

$ bkmr search 'foo-bar'
main:  Error: ... Repository error: Repository error: performing FTS search for query: foo-bar: Database error: no such column: bar
PR:    Error: ... Repository error: performing FTS search for query: foo-bar: Database error: no such column: bar

$ bkmr hsearch '  '
main:  Error: ... Query error: fts5: syntax error near ""
PR:    Error: ... Validation failed: ... hybrid search query must not be empty (use `search` to browse by tags)

Testing

Tests written first and verified failing on main:

• Error display contains Repository error: exactly once (context-wrapped Query and plain Other)
• Blank and whitespace-only hsearch queries return ApplicationError::Validation

make test green.