fix(acp): retry transient agent startup

[samiriasbeck]

Summary

• retry each ACP slot up to six times
• use bounded exponential backoff from 2 to 30 seconds
• keep a 60-second timeout for each attempt
• make attempts and retry delays shutdown-aware
• retain partial-pool behavior after the retry budget is exhausted

Why

Short-lived adapter or transport startup failures should recover without requiring a full harness restart.

Tests

• bounded retry backoff coverage
• cargo test -p buzz-acp
• cargo clippy --workspace --all-targets --all-features -- -D warnings
• just ci