docs: teach token-efficient vx agent workflows

[loonghao]

Summary

• Teach vx skills and agent-facing docs to prefer structured, filtered, and explicit compact output for token-heavy workflows.
• Sync the canonical skills/ content with .agents/skills/ compatibility copies.
• Add coverage for compact output comparison and vx ai setup installing the embedded token-efficient skills.

Validation

• vx cargo fmt --all
• vx cargo test -p vx-output-filter
• vx cargo test -p vx-cli --test ai_setup_tests
• vx git diff --check