fix: deprecate --visibility flag instead of removing it

[ToreMerkely]

Removing --visibility in #918 made passing the flag illegal (unknown
flag), breaking existing scripts. Restore it as a deprecated flag so it
is accepted with a deprecation notice rather than erroring.

Drop the hardcoded visibility "private" added in #922 as a backwards-
compat shim for servers that still required the field; prod no longer
requires it. The field is now sent only when the user explicitly passes
the deprecated flag (json omitempty), preserving compatibility with
older instances that still mandate it.

Co-Authored-By: Claude Opus 4.8 (1M context) noreply@anthropic.com

[claude]

Claude finished @ToreMerkely's task in 2m 8s —— View job

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PR Review: Deprecate --visibility flag instead of removing it

• Run static analysis (build, vet, mod tidy)
• Read changed files and review code
• Check test coverage
• Post review findings

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Static Analysis

All checks pass cleanly:

• go build ./... — compiles successfully
• go vet ./... — no issues
• go mod tidy — no dependency drift

Review Summary

This is a clean, well-scoped fix. Removing --visibility entirely in #918 turned an accepted flag into an unknown-flag error, breaking existing scripts. This PR correctly restores it as a deprecated flag with a deprecation notice.

What's good:

• The json:"visibility,omitempty" tag is the right approach — the field is only serialized when the user explicitly passes --visibility, avoiding unnecessary payload for modern servers while keeping older instances working
• The MarkDeprecated + error handling pattern matches the existing precedent in createEnvironment.go:106-109
• The test case validates both that the flag is accepted and that the deprecation notice appears
• The flag description in root.go is clearly marked [deprecated], consistent with how other flag descriptions are prefixed

One thing to verify (not blocking):
Removing the hardcoded Visibility = "private" means users on older self-hosted Kosli instances that still require the field will need to pass --visibility private explicitly (with a deprecation warning). The PR description says prod no longer requires it, so this should be fine — but worth confirming that self-hosted/on-prem users have been considered.

Minor nit: posted as an inline comment on the MarkDeprecated block — there's an existing DeprecateFlags helper in cli_utils.go that could be used, but the inline approach matches existing code, so it's a take-it-or-leave-it suggestion.

Overall: Looks good to merge. The approach is clean, the test coverage is sufficient, and it solves the backwards compatibility issue without overcomplicating things.