The completion-guard error names the failing sub-check

[clkao]

Completion failures now identify the exact broken condition, shortening diagnosis while preserving durability and terminal-gate authority.

What changed

• Emit distinct remedies for four completion failure classes.
• Refuse annotated checklist near-misses with actionable line diagnostics.
• Support consumed nonterminal completion through direct and merge-guard paths.
• Document the guarded direct terminal command.

Evidence

• 4/4 acceptance criteria passed with behavioral and on-disk-state evidence.
• 2/2 repository suites passed, including race; detached audit found no candidate defect.

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9xn