Shorten memory summaries to terse agent-facing gists #293

See #290, section 1. Title up to 80 characters for people, summary up to 300 characters as a terse gist for recall, detail for everything else.

Work

• New limits in memory_store, the web editor and the data model; tool descriptions and system guide explain the roles and show a terse example.
• Migration: a summary over 300 characters moves into an empty detail and gets a shortened summary; where detail is already filled, keep the text and let the hygiene score report it. Existing titles over 80 characters are reported, not cut.
• Batch store (Memory Batch v1) validates the same limits with a clear error.

Acceptance

• Migration is tested on SQLite and checked against SQL Server LocalDB; no text is lost.
• Store over the limit fails with a message naming the field and limit.
• Existing revisions stay readable in history.