feat(workflow): support message thread context

[wayne-o]

Summary

• expose root and parent message event IDs to workflow templates and conditions
• add a json template filter for safely embedding message text in webhook JSON bodies
• allow send_message actions to specify a templated top-level thread_root
• validate that the root exists in the workflow channel and is not itself a reply
• persist generated replies with the correct NIP-10 tag and thread metadata
• cover top-level, direct-reply, nested-reply, schema, template, and condition behavior

This provides generic workflow primitives for bridging an external conversation into a Buzz thread without adding application-specific behavior to Buzz.

Related issue

N/A. Supports the N2 Assist/Buzz conversation bridge in n2retail/n2#17296.

Testing

• ./bin/cargo fmt --all -- --check
• ./bin/cargo test -p buzz-workflow (159 passed, 2 Postgres tests ignored)
• ./bin/cargo check -p buzz-relay
• live multi-turn round trip: N2 escalation -> support question in widget -> customer answer attached to the same Buzz root -> second support reply rendered in the widget

The local Lefthook wrapper could not run because just is not installed; the formatter, workflow suite, and relay check were run directly.