Explicit --issue run can be incorrectly skipped for non-assigned issues

[0xroylee]

Source workflow reported a bug.

Linear: https://linear.app/roy/issue/ROY-24/should-we-use-sqlite-to-cache-data-or-config-for-scale-up-later
PR: #11

Details:
In processIssue, the new guard if (!existing && !isAssignedState) return skips processing (src/workflow.ts:163-171). fetchWork(issueArg) returns the targeted issue regardless of state, so piv-loop run --issue ENG-123 can now no-op unless the issue is currently in the assigned state or has prior local run-state. This is a behavior regression for manual/retry workflows and needs an exception for explicit issue targeting.