Trim MCP responses: no nulls, relaxed escaping, lean task entries, current memory revision #306

Follow-up to #303 after measuring task_list by field: key names were half the characters, timestamps a fifth.

• All tool results go through McpJson: null fields are left out, and the relaxed encoder writes backticks, quotes and umlauts as themselves instead of six-character escapes.
• task_list entries and item_get subtasks: no timestamps and no assignee reference; normal priority and zero counts left out. age (today, 3d) only for changes within seven days, 7d+ for tasks still in_progress.
• proposal_get queue: no times, false flags left out. item_search memory hits leave out Archived when false.
• item_get on a memory returns the entry plus its current revision flat (RevisionId, Title, Summary, Detail, Tags, AuthorName) and only the ids of pending revisions; item_history keeps returning all revisions.
• Guide v30 explains omitted fields and age.

Measured on this project's 29 open tasks (o200k tokens): 3,111 tokens with #303 entries, about 950 now.