feat: multiline composer with undo and yank recovery

[AetherAI3]

Closes #321.

Change

• Raw TTY Ctrl+J inserts LF, while Enter/CR submits. lfSubmits explicitly restores LF submission for terminals that cannot distinguish the keys; line mode remains unchanged.
• Bounded draft undo (64 snapshots, 1 MiB of snapshots) restores content and cursor. Adjacent typing coalesces; bracketed paste is one edit. Ctrl+Y restores the last killed text, capped at 1 MiB.
• Submission, clear, and input-owner handoffs end recovery. Approval, setup, terminal, shell review, and queued-draft ownership retain their existing authority.
• The single-row renderer keeps ⏎ and adds a compact line-count hint. README and COMMANDS document behavior and terminal ambiguity.

Evidence

• npm run docs:check (includes build): passed; six generated docs checked.
• Focused Node test set for composer, config, chat keys, queue/streaming, shell, input buffer, render and decoding: 100 passed, 0 failed, 4 pre-existing shell fixtures skipped on this host.
• git diff --check: passed.
• PR Preserve exact shell review and queue authority on v4.21.0 main #309 coordination: src/commands/chat.ts auto-merges with its current draft head. A docs/generated/commands.md conflict already exists between PR Preserve exact shell review and queue authority on v4.21.0 main #309 and origin/main; comparing PR Preserve exact shell review and queue authority on v4.21.0 main #309 against this branch yields the same sole conflict. No shell-review or queue-authority code was changed here.