Add project webhook CLI support

[plutoless]

Summary

• Add project webhook events/list/show/create/update/delete commands for rtc, rtm, and convoai features.
• Add webhook API adapter, validation, generated/default secrets, delivery-region defaults, read-merge-update behavior, pretty/JSON rendering, and MCP tool coverage.
• Document webhook automation contracts, command docs, llms surface, and structured error codes.

Test Plan

• go run ./cmd/gendocs -check
• ./scripts/check-error-codes.sh
• git diff --check
• go test ./...
• go build -o agora .
• ./agora --help --all --json