fix: allow postflight triage with pending objectives

[cavvy-dev]

Problem

During postflight planning, next can prescribe plan triage while triage refuses
with “pending behind the current objective backlog.” The canonical queue treats
newly planned objectives as backlog in this lifecycle phase, but the triage-start
guard still treats a retained plan-start score as evidence of active execution.

This addresses the triage contradiction reported in #787, not the separate
recovered-item scan-gate behavior described there.

Fix

Honor explicitly persisted planning mode when evaluating the objective execution
guard. Retain the guard for execute-mode and legacy plans without a persisted
phase, and retain the separate unconfirmed-stage-report guard.

No objectives are resolved, skipped, or removed by this change.

Verification

• New regression before fix: 1 failed, 3 passed.
• Focused lifecycle/queue tests after fix: 39 passed.
• Clean upstream checkout plus this patch: python -m pytest desloppify/tests/ -q
  — 5814 passed, 5 skipped.
• Read-only evaluation against the affected saved plan now permits its prescribed
  planning action while retaining its 128 pending objective findings.
• The prescribed triage runner now starts without an attested override.

The initial sandboxed full-suite attempt could not download required parser
assets. Re-running with network access passed; no tests were disabled.