fix(tui): clear stale suggestion prefill after turns

[edwin-zvs]

Summary

• clear the TUI suggestion keyword draft at turn completion and when the session returns to awaiting input
• prevent stale adapter editor snapshots from repopulating Generate keywords
• document the turn-boundary behavior and add a regression test

Validation

• cargo test -p construct-cli --bin construct suggestion_draft_clears_at_turn_completion
• cargo test -p construct-cli --bin construct suggest_deck (19 passed)
• cargo build (passed; relevant binary: construct)
• full CLI tests: 1234 passed, 1 unrelated pre-existing color backend test failed (color::tests::truecolor_backend_still_emits_rgb)

Relevant binary: target/debug/construct.