Port channel shared-object materialization

[madtank]

Summary

• ports the useful slice from stale PR Materialize shared objects in channel prompts #66 onto current main
• materializes forwarded task/context/card metadata into channel prompts so live agents can see what was shared
• includes attachment metadata and suggested axctl inspection commands for context-backed attachments

Supersedes #66. The original branch is stale and would delete current Gateway code if merged as-is.

Verification

• uv run ruff check ax_cli/ tests/test_channel.py
• uv run ruff format --check ax_cli/ tests/test_channel.py
• uv run pytest -q tests/test_channel.py
• uv run pytest -q
• git diff --check