chore: update mcpcall references from 0.3.0 to 0.4.0

[loonghao]

Summary

• Update all mcpcall version references from 0.3.0 to 0.4.0 across documentation, skill files, provider comments, and tests
• mcpcall v0.4.0 was released on 2026-05-25 with new features (discover mcp configs and package release zips)
• The provider's fetch_versions_with_tag_prefix already resolves v0.4.0 dynamically, but hardcoded version references in docs/skills/tests were stale

Changed files

• skills/vx-usage/SKILL.md (and all agent skill mirrors)
• docs/tools/ai.md and docs/zh/tools/ai.md
• crates/vx-providers/mcpcall/provider.star (comment)
• crates/vx-starlark/tests/mcpcall_tests.rs

Test plan

• cargo test -p vx-starlark --test mcpcall_tests — all 4 tests pass with v0.4.0
• cargo test -p vx-starlark --test lint_all_providers_test — 142/142 providers clean
• cargo fmt --all -- --check — no formatting issues
• cargo clippy --workspace --all-targets -- -D warnings — no warnings
• cargo test --workspace — full test suite passes