Require ChatGPT Plus and remove obsolete closeout workflow

[referencefield]

Development-only product-contract cleanup; this does not start the acceptance gate.

Changes:

• set ChatGPT Plus as the minimum supported ChatGPT plan; Free and ChatGPT Go are unsupported
• require @GitHub to be installed/selected, authenticated to GitHub, authorized for the exact working repository, and expose repository read/write actions before READY
• keep higher plans supported only when that required GitHub capability is actually available
• remove the obsolete end-of-session persistence workflow entirely; persistence remains point-of-change through the existing persistence watch and explicit routing controls
• replace the former E-55 with a GitHub-visible-but-not-authenticated readiness scenario
• make Gate 3 test the normal ChatGPT Plus user path
• add plan/plugin state to bug reports
• make the structural validator enforce the support contract and reject regression to the removed workflow or generic paid-plan wording

protocol_version remains unreleased and protocol_status remains development.