fix: harden release-please and add mcpcall

[loonghao]

Summary

• skip stale release-please update-pr runs so delayed jobs do not recreate already-merged release PRs
• tighten release PR gating to reject incomplete changelog-only/version-only PRs and require Cargo.lock with version bumps
• add the mcpcall v0.3.0 provider and document compact vx --compact mcpcall ... --json usage for agents/CI
• correct vx metrics percentiles/stage percentages and vx metrics tokens run/token accounting

Validation

• vx cargo test -p vx-metrics
• vx cargo test -p vx-starlark --test mcpcall_tests -- --nocapture
• vx cargo test -p vx-starlark --test lint_all_providers_test -- --nocapture
• vx actionlint .github/workflows/release-please.yml .github/workflows/ci-release-pr.yml
• vx git diff --check
• vx cargo build -p vx
• ./target/debug/vx.exe --compact mcpcall@0.3.0 --version
• isolated VX_HOME smoke for metrics --json and metrics tokens --json
• vx just quick