ci: standalone-fs smoke test against latest greptimedb release

[sunng87]

What

Adds a GitHub Actions workflow (.github/workflows/ci.yml) + ci/smoke.sh that runs on every push/PR:

1. Download the latest GreptimeTeam/greptimedb release binary (GNU linux-amd64).
2. Start the testbed's standalone-fs server via process-compose inside nix develop (the testbed's own machinery — no garage/etcd needed).
3. Assert that SQL works: write a row and read it back through the client PostgreSQL port (testbedctl psql).

Uses DeterminateSystems/nix-installer-action + magic-nix-cache-action so the nix develop devShell is cached across runs.

Validated locally

Ran the exact CI path with the v1.1.3 release binary:

GreptimeDB healthy after 1s
read-back value: 42
SMOKE TEST PASSED

⚠️ Temporary step: strip iceberg_manifest plugin

config/standalone-fs.toml enables the iceberg_manifest plugin, which is newer than the latest release — the release hard-fails with:

enum PluginOptions does not have variant constructor iceberg_manifest

So a workflow step strips that plugin block for now. Once the greptimedb patch to tolerate unknown plugin options lands, that step should be deleted (it's clearly commented in the workflow). The full testbed config will then work against the release as-is.

Notes

• Failures dump standalone-fs logs; teardown runs on always.
• Branches/PRs cancelled on supersede via concurrency.

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Generated via testbed session.