[codex] clarify registered agent dispatch

[chaochaoweb3]

Summary

• replace ambiguous relative agent prompt references in the understand skill with explicit registered subagent_type names
• keep plugin-root prompt paths only as fallback instructions for hosts that cannot dispatch plugin agents by type
• cover project-scanner, file-analyzer, assemble-reviewer, architecture-analyzer, tour-builder, and graph-reviewer dispatches

Fixes #464

Validation

• rg confirmed no remaining dispatch instructions point at bare agents/*.md paths
• git diff --check