blindspot: save reports under correspondence/blindspot

[nsmiller2501]

Summary

This PR makes /blindspot persist its audit reports under correspondence/blindspot/ while still surfacing the report inline.

What changed

• Adds Write and mkdir permissions needed to create report artifacts.
• Lazily creates correspondence/blindspot/ when needed.
• Writes each Blindspot Report to correspondence/blindspot/YYYY-MM-DD_<output-slug>.md.
• Adds a README explaining the skill, its four-quadrant audit, and how it complements /referee2.

Why

This follows the same project-correspondence convention used by other audit workflows: the user can react inline during the session, while the report remains available as a durable project artifact.

Testing

• Reviewed the branch diff against the PR notes.
• No automated tests were run; this is a skill/documentation protocol update.