Session retry loops forever — no max attempt cap or configurable limit #25733

Description

SessionRetry.policy() in retry.ts retries retryable errors indefinitely with no maximum attempt count. The policy() function stops only when retryable() returns undefined (non-retryable error). Once an error is classified as retryable, the schedule retries forever.

This is the same underlying problem described in #21960, #17648, and #12234.

Steps to reproduce

1. Use a provider that returns persistent 429/overloaded errors (e.g., GitHub Copilot weekly limit exhausted)
2. Send any message
3. Observe the session retrying forever with exponential backoff

Expected behavior

• Configurable max retry attempts (e.g., experimental.max_retries)
• After max attempts exhausted, session should stop and surface the error
• Backward compatible: existing behavior preserved when not configured

Suggested fix location

• packages/opencode/src/config/config.ts - add experimental.max_retries to schema
• packages/opencode/src/session/retry.ts - add max attempt check in policy()