Fix GitHub synchronization after Planfile ticket archival #8

Reproduction: complete a Planfile ticket with an existing GitHub sync mapping. Planfile moves it from current.yaml to a history sprint; planfile sync github --direction to then reports no local tickets and leaves the GitHub issue open.\n\nAcceptance criteria:\n1. Sync discovers mapped tickets in active and archived/history sprints.\n2. A completed ticket closes its mapped GitHub issue.\n3. A dry run reports the intended close without mutating GitHub.\n4. Add a regression test covering the archival path.

Reproduction: complete a Planfile ticket with an existing GitHub sync mapping. Planfile moves it from current.yaml to a history sprint; planfile sync github --direction to then reports no local tickets and leaves the GitHub issue open.\n\nAcceptance criteria:\n1. Sync discovers mapped tickets in active and archived/history sprints.\n2. A completed ticket closes its mapped GitHub issue.\n3. A dry run reports the intended close without mutating GitHub.\n4. Add a regression test covering the archival path.