strategy: also resolve unreadable declared task transcripts through the inventory

The inventory lookup ran only for records whose declared path was empty or missing, so a declared file that exists but cannot be opened (e.g. EACCES) skipped it and the task was stored as "transcript unreadable".

agent.CheckTranscriptReadable opens and closes an external transcript path without reading it (stat-only under .entire, matching StatTranscriptFile), and resolveInventoryTaskTranscripts uses it in place of the existence check.

Also update the task-record resolution order in checkpoint-implementation.md, which still read "declared path first, agent-layout fallback" (Copilot review on #2645).