feat: surface duplicate context groups

[cbusillo]

Summary

• add duplicate grouping to the context ledger using stable duplicate keys
• show top possible duplicate groups in /context before the source table
• remove an unused test-only clipboard helper that produced a warning in focused TUI tests

Validation

• cargo test -p code-core context_ledger::tests::duplicate_groups_collect_repeated_keys_by_weight
• cargo test -p code-tui context_ledger_display_surfaces_duplicate_groups
• ./build-fast.sh

Refs #92