Clarify or enforce sharing behavior in archive mode

[backnotprop]

Problem

CLAUDE.md and AGENTS.md describe standalone archive mode as opening a read-only archive viewer with “sharing disabled,” but the archive server currently exposes a working share path.

This was confirmed during the independent review of #1171 by running both Bun and Pi archive runtimes and testing their server behavior. #1171 correctly blocks mutation; it does not disable sharing.

Decision needed

Choose and make the product/docs consistent:

• If sharing is intended in archive mode, correct both repository operating documents and verify public documentation uses the same wording.
• If sharing should be disabled, enforce that server-side in both Bun and Pi runtimes and add parity tests.

Context

• Review: fix(annotate): enforce archive read-only surfaces #1171 (comment)
• Current claim: Browser opens read-only archive viewer (sharing disabled)
• This is pre-existing and is not a blocker for fix(annotate): enforce archive read-only surfaces #1171.