Add structured TUI chat mode

[edwin-zvs]

Summary

• Rename the TUI's structured transcript view to Chat Mode and use it as the default for headless/non-PTY sessions.
• Make Chat Mode structured-event-only: skip PTY bytes/resizes/editor state/agent-status and show an empty-state hint when no chat events exist.
• Add a normative spec for TUI Chat Mode and regression tests for PTY exclusion and bootstrap-message filtering.

Tests

• cargo test -p agentd-cli chat_mode -- --nocapture
• git diff --check
• cargo check -p agentd-cli
• cargo build

Binaries

Built by cargo build in this worktree:

• /Users/moon/agentd/.claude/worktrees/tui-chat-mode/target/debug/agent
• /Users/moon/agentd/.claude/worktrees/tui-chat-mode/target/debug/agentd
• /Users/moon/agentd/.claude/worktrees/tui-chat-mode/target/debug/agentd-mcp
• /Users/moon/agentd/.claude/worktrees/tui-chat-mode/target/debug/agentd-adapter-claude
• /Users/moon/agentd/.claude/worktrees/tui-chat-mode/target/debug/agentd-adapter-codex
• /Users/moon/agentd/.claude/worktrees/tui-chat-mode/target/debug/agentd-adapter-zarvis

Relevant binary for this PR: agent (changes are in crates/cli).