Add release smoke-check script

[Yashu-Maurya]

Related to #31.

Release checks are manual at the moment, so I put together a small smoke-check script to catch broken builds before publishing.

What's in here:

• scripts/smoke-check.sh: builds the binary, starts serve on 127.0.0.1:18070, polls /health until it's ready (30s default), then shuts everything down. Exits non-zero with the server log tail if it never becomes healthy. SMOKE_HOST/SMOKE_PORT/SMOKE_TIMEOUT/SMOKE_BINARY can override the defaults.
• Documented it in docs/CONTRIBUTING.md under a short "Release smoke check" section.

Kept it source-build focused like the issue asks, Docker and go install checks left as follow-ups.

How I tested it:

• make build then ./scripts/smoke-check.sh from a clean checkout: healthy, exit 0, no leftover server process.
• Pointed it at an occupied port: fails fast with exit 1 and prints the bind error from the log.
• go vet ./... clean, unit tests (cmd, core, extract, google, bing) pass. No Go code touched so make fmt is a no-op.