feat: add temporal gateway state

[yairfalse]

Summary

• add bounded temporal API types for events, compact snapshot history, and semantic gateway diffs
• keep recent in-memory observations in LocalGatewayQuery and expose /api/v1/timeline and /api/v1/diff
• thread timeline, diff, and diagnose --since-seconds through CLI, remote client, and MCP tools

Validation

• cargo fmt --all -- --check
• cargo test --workspace
• cargo clippy --workspace --all-targets -- -D warnings
• teko verify --claim journal --claim snapshots --claim diff --claim diagnose-since --json
• teko close --json