Fail fast if repo storage_path does not exist on disk, before
blocking the thread pool with spawn_blocking. This prevents
invalid tasks from consuming blocking threads while failing.
- Fix match pattern: `Ok(Ok(Err(_)))` must be treated as NOT a panic,
since execute_task_body returns Err(()) on task failure (not a panic).
Previously the Err path incorrectly set panicked=true, which could
cause pool to appear unhealthy.
- Add "task failed" log before retry/discard decision so real error
is visible in logs (previously only last_error on exhausted-retries
path was logged, which required hitting 5 retries to see the cause).
- Convert 3 remaining pool/mod.rs shorthand logs to format!() pattern.
- Add retry_count to HookTask with serde(default) for backwards compat
- Limit hook task retries to MAX_RETRIES=5, discard after limit to prevent
infinite requeue loops that caused 'task nack'd and requeued' log spam
- Add nak_with_retry() in RedisConsumer to requeue with incremented count
- Standardize all slog logs: replace "info!(l, "msg"; "k" => v)" shorthand
with "info!(l, "{}", format!("msg k={}", v))" across ssh/authz.rs,
ssh/handle.rs, ssh/server.rs, hook/webhook_dispatch.rs, hook/pool/mod.rs
