fix(jira): prevent CLI webhook-secret updates from losing refreshed OAuth tokens

[ayushtr-aws]

Problem

bgagent jira setup and bgagent jira update-webhook-secret read the shared per-tenant OAuth bundle, modify webhook_signing_secret, and write the complete bundle back. A concurrent Lambda refresh in cdk/src/handlers/shared/jira-oauth-resolver.ts can rotate refresh_token between the CLI read and write, allowing the CLI to overwrite the newly rotated token with a stale value and break subsequent Jira authentication. The existing invite-user update path has the same read-modify-write pattern.

This was identified as a non-blocking review finding on #710. It is intentionally separate because Secrets Manager PutSecretValue does not provide a simple version-conditioned compare-and-swap; ClientRequestToken provides idempotency, not protection against overwriting a concurrently created version.

Acceptance criteria

• Define a concurrency-safe ownership/update strategy for mutable Jira OAuth bundles.
• Prevent CLI updates from replacing a refresh_token rotated after the CLI read.
• Preserve webhook and Forge app-actor fields across OAuth refresh and CLI updates.
• Add a test that interleaves a Lambda refresh with each affected CLI update path.
• Document retry/recovery behavior when a concurrent update is detected.