feat: print the exact resume command when a task pauses

[msampathkumar]

Goal: When a task pauses and needs the caller to act, hand them the exact command to continue — no guesswork.

User story: As a developer (or an AI agent) whose task paused for input, I want the CLI to print the exact a2a command that resumes it, so that I can continue the task by copy-paste instead of assembling --task-id/--context-id flags myself.

Priority: P2 · Area: CLI code

Problem

The CLI is stateless — it remembers nothing between invocations, so the caller must supply the task/context identifiers to continue an interaction. When a task pauses (INPUT_REQUIRED or AUTH_REQUIRED), the identifiers are printed, but the caller still has to know which command and flags to run next and assemble them by hand.

Proposed behavior

When a run stops on an interrupted state, print a ready-to-run resume line, e.g.:

State:   INPUT_REQUIRED
Task:    01a0-...
Context: 01a0-...
Resume:  a2a send --task-id 01a0-... "<your reply>"

• Show it in human-readable (text) output on both INPUT_REQUIRED and AUTH_REQUIRED.
• Include the identifiers the next call needs; keep it copy-pasteable.
• -o json is unchanged (the identifiers are already in the structured output).

Who benefits

• Developers driving a multi-turn interaction from the terminal.
• AI coding agents that need an unambiguous next command rather than inferring flags.

Acceptance criteria

• On INPUT_REQUIRED and AUTH_REQUIRED, text output includes a Resume: line with a valid command.
• The printed command carries the correct --task-id (and --context-id when relevant).
• -o json output is unaffected.

Environment

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

Notes

Related work may already be in flight in PR #23 — coordinate to avoid duplication.

[msampathkumar]

Implemented in #23 — formatResumeHint prints a copy-pasteable a2a send --task-id <id> "<reply>" for INPUT_REQUIRED / AUTH_REQUIRED in text mode, with a test. This should close when #23 merges.