Make implementation private to the Issue Run

[rochecompaan]

Parent

#154

What to build

Internalize fresh and resumed implementation, workflow-specific Pi invocation and result interpretation, blockers, repair, todo validation, and implementation persistence under the Issue Run owner.

Acceptance criteria

• Implementation is no longer an independently supported public stage and does not expose callback plumbing as a replacement interface.
• Fresh and restored implementation preserve Pi sessions, resource-profile selection, saved results, repair behavior, and todo validation.
• Pi blockers preserve their labels, comments, questions, persisted state, and explicit outcome; malformed and unexpected results preserve current fault behavior.
• Successful implementation persistence and high-risk progress, Host, command, and state-write traces remain compatible.
• Meaningful implementation behavior is covered through runOneIssue before redundant direct-stage coverage is removed.

Blocked by

• Make development-environment preparation private to the Issue Run #161 — Make development-environment preparation private to the Issue Run