feat: support @ mentions in chat message reply

[mvanhorn]

Summary

dws chat message reply now accepts --at-open-dingtalk-ids <csv> and --at-all, forwarding atOpenDingTalkIds / isAtAll to the upstream send_personal_message tool. The reply content goes through the same marshalMessageContent path as chat message send, so <@...> placeholders reach DingTalk clients unescaped. Flag help, both skill reference docs, and CHANGELOG.md document that --text must contain matching <@openDingTalkId> / <@all> placeholders.

This closes the gap in #359: an agent that polls chat message list-mentions and replies in-thread had no way to notify the original sender, even though the upstream tool schema already supports these fields (confirmed in #310). The implementation follows the landing plan PeterGuy326 laid out in the issue thread: pure CLI passthrough mirroring the send command's flag names and splitCSVStrings handling, with no changes to the bot/webhook send paths.

Verification

• make build
• make lint
• make test
• make policy
• ./scripts/policy/check-generated-drift.sh
• ./scripts/policy/check-command-surface.sh --strict (command surface changed: two new reply flags)

Notes

Table-driven tests cover ids-only, at-all, no-flags (params identical to current behavior), and whitespace-only ids. On make test, the cli, contract, and mock_mcp suites pass; the integration/extensions, integration/recovery, and scripts suites fail identically on a clean upstream/main checkout in this environment, so those failures are pre-existing and unrelated to this change. No risks beyond the new optional flags; behavior without them is unchanged.

Fixes #359