Add CLI stale-version warnings

[krandder]

Summary

• publish a public CLI version manifest from the API at /v1/cli/version
• add a cached CLI startup check that warns when the installed CLI is stale
• make the installer output mention futarchy update

Why

Users can already upgrade with futarchy update, but today the CLI does not tell them when they are behind. That means people only discover they are stale after hitting compatibility bugs.

Behavior

• the API now reports the latest CLI version and update command
• the CLI checks that endpoint at most once per day and caches the result locally
• interactive CLI runs print a warning to stderr when the installed version is older than the latest published version
• --json runs stay quiet to avoid polluting machine-readable output

Validation

• python3 -m py_compile core/api.py core/api_models.py core/test_api.py core/test_cli_update_check.py cli/futarchy_cli/api.py cli/futarchy_cli/main.py cli/futarchy_cli/update_check.py
• pytest -q core/test_api.py core/test_cli_update_check.py
• bash -n static/install.sh
• python3 -m futarchy_cli.main update --help

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