feat: add interactive chat for multi-turn agent sessions

[msampathkumar]

Goal: Give a human a simple, interactive multi-turn conversation with an agent from the terminal, without re-typing identifiers on every turn.
User story: As a developer exploring an agent, I want an interactive chat that remembers the conversation, so that I can go back and forth without re-passing --context-id and --task-id each turn.

Priority: P2 · Area: CLI code

Problem

Driving a multi-turn conversation today means running a2a send repeatedly and manually carrying --context-id (and --task-id while a task is paused) from one command's output into the next. There is no interactive session command:

$ a2a chat --help
Error: unknown command "chat" for "a2a"

Proposed behavior

Add an interactive a2a chat command for a back-and-forth session with one agent:

a2a chat -a <agent>

• Reads a message from the prompt, sends it, prints the agent's reply, and loops.
• Carries the contextId across turns automatically (and the active taskId while a task is interrupted), so the user never copies identifiers by hand — the CLI stays stateless between processes, but a live chat session holds this in memory for its own duration only.
• On INPUT_REQUIRED, prompts for the reply and continues the same task; on AUTH_REQUIRED, stops and explains what's needed.
• Prints the taskId/contextId so a user can resume later with a2a send.
• Plain, readable output; exits cleanly on EOF/Ctrl-D or a /quit command.

Interactive-safety requirements

• chat is interactive-only: it must auto-degrade to non-interactive send behavior when stdout is not a terminal, and must never block waiting on input in a pipe/CI context.
• It must not be the default command; a script's behavior must not change because it happened to run in a terminal.

Who benefits

• Developers exploring or debugging an agent conversationally.
• Anyone giving a quick demo of an agent from the terminal.

Acceptance criteria

• a2a chat -a <agent> runs a multi-turn loop, carrying contextId/taskId automatically.
• Handles INPUT_REQUIRED (prompt + continue) and AUTH_REQUIRED (stop + explain).
• Auto-degrades / does not block when stdout is not a terminal.
• /quit and EOF exit cleanly; the last taskId/contextId are printed for later resume.

Environment

• CLI version: a2a version v0.0.0-…1e29dfe94f95+dirty
• OS: Linux x86_64

Notes

This is an ergonomics layer built entirely on top of the existing send/task behavior — it introduces no new protocol behavior, only an interactive front-end that manages identifiers for the length of the session.