Recover markerless assignment PRs safely

[morganmcg1]

Summary

• add an explicit advisor/operator-only adopt_assignment transition for an existing markerless assignment PR
• verify the authenticated PR author, configured routing, exact branch point, remote head ancestry, and absence of conflicting protocol state before and after adoption
• make successful adoption idempotent when a student legitimately advances the branch
• emit malformed_assignment to advisors and count only marker-valid assignments as active or duplicate work
• expose create-assignment and adopt-assignment through one operator CLI that calls the same typed executors as OpenHands
• clarify that repair_assignment_routing repairs labels and draft state only; it cannot create assignment identity

Root cause

An operator-created PR could receive the normal student and WIP labels while bypassing create_assignment, leaving it without the canonical assignment marker. Students detected the invalid state, but advisors neither received that failure nor had a safe operation to adopt the PR. Raw labels also made the malformed PR appear to occupy a healthy student slot.

This change keeps the harness boundary narrow: it validates identity and routing, reports invalid state, and offers one explicit repair operation. It does not infer assignments from prose or automatically mutate malformed PRs.

Safety and recovery behavior

• adoption is advisor-only and restricted to configured students and the configured advisor base
• the PR must be authored by the authenticated actor, open, draft, WIP, uniquely routed, and markerless
• the supplied base must be the exact merge base of the advisor and assignment branches, not merely an older ancestor
• the expected head may be an ancestor only when reconciling a replay after legitimate student progress; the first marker write still requires the exact current PR head
• remote Git history is checked both before and after the marker transition
• terminal result/disposition state and conflicting markers fail closed
• the existing human-readable PR body and unrelated labels are preserved
• GitHub PR body writes have no lease in this client, so concurrent body edits remain explicitly unsupported and documented

Validation

• uv run --extra dev pytest -q --maxfail=1 — 926 passed, 6 skipped
• git diff --check
• python -m compileall -q senpai_agent

This PR is intentionally not rolled out to any live advisor or student deployment. It also does not perform the one-time repair for the existing Maple PRs.