Materialize shared objects in channel prompts

[madtank]

Summary

• Materialize metadata.forward into Claude/channel prompt text so live agents can see whether a task, context item, message, or card was shared.
• Include suggested axctl follow-up commands for shared tasks/context where possible.
• Include attachment metadata from top-level and metadata-backed attachment fields.

QA

• uv run ruff check ax_cli/commands/channel.py tests/test_channel.py
• uv run pytest tests/test_channel.py -q

Notes

• This is the small replacement for PR Add Activity Stream QA fixtures and channel share materialization #64.
• Live channel agents need restart/reload to pick up this behavior.