Make update notice prominent on every non-JSON command (#269)

[sdairs]

Closes #269

Problem

Users weren't updating clickhousectl because the update notice was barely surfaced — it only printed on --help/--version, never during normal command execution.

What changed

• Show the notice on every non---json command. The cached update notice now prints to stderr at the very end of every command that produces human-readable output. --json and detected coding agents continue to suppress it, so machine/agent consumers are unaffected.
• New single-line message:

  There is a new version of clickhousectl. Update with clickhousectl update.

• --version always checks the network, refreshes the cache, and resets the staleness timer, then prints the notice from the fresh cache.
• update --check now also writes the cache and resets the timer on every run.
• Successful update clears the cache so the notice disappears immediately.
• Kept the 24h background-refresh gate for normal commands (one network call per day). On network failure we now preserve any previously-cached "update available" version instead of clobbering it with the current version, so a known update is never hidden.

Tests

• cache_is_stale boundary cases (update.rs).
• command_json_flag mapping + --json/update suppression (main.rs).
• All 353 tests pass; clippy clean.

Manual verification (live network, current latest 0.3.1)

Scenario	Result
--version, seeded stale cache	Hit network, cache rewritten to 0.3.1, timer reset
--version on a temp v0.0.1 build	Printed version, then the notice
update --check, seeded wrong cache	Update available: v0.0.1 → v0.3.1; cache rewritten, timer reset
Normal human command, newer cache	Notice prints at the very end after output
Same command with --json / under agent detection	Notice suppressed

🤖 Generated with Claude Code

[sdairs]

bugbot review

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