build: hoist RUSTFLAGS=--cfg tokio_unstable into workspace cargo config

[Richard1048576]

Summary

The aptos-core fork that gravity_node and gravity_cli depend on uses several tokio APIs gated behind `#[cfg(tokio_unstable)]` — `RuntimeMetrics`, tokio-console integration, named tasks. Dozens of files reference these. Without the cfg flag the build fails with `unresolved import: this item is gated behind a cfg(tokio_unstable) flag`.

Until now the flag has been repeated at every build entry point:

Where	How
`.github/workflows/release.yml`	`env: RUSTFLAGS:`
`.github/workflows/rust-ci.yml`	`env: RUSTFLAGS:`
`.github/workflows/unit-test.yml`	`env: RUSTFLAGS:`
`Makefile`	two targets ahead of the line, two missing it
`cluster/deploy.sh`	inline per-cargo-build call
`.agents/*` docs / `cluster/docs/gcp-secret-manager.md` / `bin/gravity_cli/README.md`	manual one-liner reminder

Why this matters now

This sprawl bit two operators in the past week:

1. A team member building gravity_cli manually forgot the flag. The build failed, but a | tail in their wrapper masked cargo's non-zero exit (bash default behavior; you'd need `set -o pipefail` to catch it). They saw "looks fine" output and proceeded with a binary that wasn't actually built. Took a while to notice.

2. The Makefile is internally inconsistent. The `bench` and `kvstore` targets transitively depend on `gaptos.workspace` and `aptos-*`, so they DO need `tokio_unstable`. But their Makefile lines don't set `RUSTFLAGS`. Anyone running `make bench` from a clean checkout today gets a compile failure. Existing local Makefile runs probably work only because the operator had `RUSTFLAGS` in their shell already.

Fix

Workspace-level `.cargo/config.toml` with:

```toml
[build]
rustflags = ["--cfg", "tokio_unstable"]
```

Any `cargo build / test / check / clippy` invocation, in CI or on a fresh checkout, picks the flag up automatically.

Compatibility

Cargo's documented merge order: an explicit `RUSTFLAGS` env var takes precedence over the config file. So the existing per-workflow `RUSTFLAGS` declarations keep working unchanged — they're now belt-and-suspenders rather than the only line of defense. No CI behavior changes.

Scope

This PR is intentionally minimal: add the config, don't touch the duplicate `RUSTFLAGS` sites. Cleanup of the redundant lines can land in a follow-up once this config has soaked through a few CI runs and is proven to be the source of truth.

Test plan

• CI green on this PR (existing workflows still build)
• `unset RUSTFLAGS && cargo build -p gravity_node` succeeds on a fresh clone (was failing before)
• `make bench` succeeds without the operator having to set RUSTFLAGS in their shell (was implicitly broken before)
• No change in output binary for gravity_node / gravity_cli (only the entry-point env handling differs)

🤖 Generated with Claude Code