feat: warn when src-cli is older than the instance's recommended version

[Montana]

Hey @burmudar,

This is superseded by my last PR. I've consolidated the implementation, the main() wiring, and these tests into a single PR that builds. Splitting the tests out on their own was my mistake; on its own that branch didn't compile. I've run the go checks locally (build and tests on 1.26.4) and they pass without error.

What's in it:

• The check is invoked from main() via maybeWarnVersion, and reuses the existing getRecommendedVersion helper against /.api/src-cli/version rather than duplicating the request.
• Fail-open on everything. Missing config, network errors, an unreachable instance, or an unparseable version all result in no warning, never an error.
• 3s timeout so it never noticeably delays a command, and it writes only to stderr so --json output is unaffected.
• Skips version, help, and no-arg invocations, dev builds (version.BuildTag == version.DefaultBuildTag), and any run with SRC_SKIP_VERSION_CHECK set.
• Comparison is major/minor/patch only, so prereleases don't trigger spurious warnings.
• Unit tests cover parseVersion, isOlderVersion, and skipVersionCheck, including the prerelease and fail-open cases and each skip path.

One design note worth flagging: the check runs before flag parsing, so -endpoint and -config passed as flags aren't picked up by the early config read. The endpoint resolves from SRC_ENDPOINT, the config file, or the default. I kept it there to preserve the fail-open behavior, but happy to move the call after config resolution if you'd rather flags be honored.

I'll close #1339 and #1343 as superseded. PTAL when you have a chance.

Cheers,
Michael