feat: add agent task report endpoint

[joryirving]

Parent: #392

Add a report endpoint for harnesses to send the result of one Dispatch task.

Report outcomes:

• PR opened
• PR updated
• issue updated
• issue closed
• blocked
• failed
• no changes needed

Acceptance:

• report updates Dispatch state consistently
• report can link issue and PR numbers
• worker reports do not trigger another task
• groomer reports can update issue status/lane metadata
• errors are visible in Dispatch for later follow-up