feat: add MCP tool annotations for spec compliance

[rahulcrl]

Summary

• Adds readOnlyHint, destructiveHint, idempotentHint, and openWorldHint annotations to all 15 tools per the MCP spec
• All read tools: readOnlyHint: true, openWorldHint: false
• Write tools: openWorldHint: false with per-tool destructiveHint and idempotentHint matching their semantics
• Aligns tool annotations with the MCP spec so clients render correct read-only, destructive, idempotent, and closed-world hints:
  • readOnlyHint - Whether the tool only reads and never modifies its environment.
  • destructiveHint - For write tools, whether the update can overwrite or remove existing data (true) versus being purely additive (false). Meaningful only when readOnlyHint is false.
  • idempotentHint - Whether calling the tool again with the same arguments has no additional effect on the environment. Meaningful only when readOnlyHint is false.
  • openWorldHint - Whether the tool interacts with an open, external world (like web search) versus a closed, well-defined domain.

Annotation matrix

Tool	readOnly	destructive	idempotent	openWorld
list_databases	true	n/a	n/a	false
list_tables	true	n/a	n/a	false
get_table_schema	true	n/a	n/a	false
show_running_queries	true	n/a	n/a	false
get_cluster	true	n/a	n/a	false
list_sql_users	true	n/a	n/a	false
list_cluster_nodes	true	n/a	n/a	false
select_query	true	n/a	n/a	false
explain_query	true	n/a	n/a	false
show_statement	true	n/a	n/a	false
create_database	false	false	false	false
create_table	false	false	false	false
insert_rows	false	true	false	false
update_rows	false	true	false	false
delete_rows	false	true	true	false

Test plan

• go test ./... passes (all 8 packages)
• New subtests verify every annotation field per tool

[cockroachlabs-cla-agent]

All committers have signed the CLA.