Bug/DoS: append_journal has no size or count limits

[dkritarth]

src/mcp/tools.ts handleAppendJournal validates note/server are non-empty strings and tags is string array, but no cap on:

• note length
• number of tags / tag length
• server field length

A held write token (or compromised agent) can write unbounded-size entries, inflating KV storage cost and journal:index size indefinitely. Suggest adding sane max lengths (e.g. note <= 10000 chars, tags <= 20 items) enforced in handleAppendJournal before appendJournalEntry is called.