Add configurable retry backoff strategy for agent handoff

[Manavarya09]

Feature Request

Problem

When Claude Code hits a rate limit and Relay hands off to another agent, the retry timing is fixed. Different providers have different rate limit windows.

Proposed Solution

Add a configurable backoff strategy:

• Linear backoff — wait N seconds between retries
• Exponential backoff — double the wait time each attempt
• Provider-specific — custom timing per agent (Codex, Gemini, etc.)

Example Config

[retry]
strategy = "exponential"
initial_delay = 5
max_delay = 120
max_retries = 5

Benefits

• Reduces unnecessary API calls during rate limits
• Respects provider-specific rate limit windows
• Improves reliability for long-running sessions