Remove legacy basectl ci command

[codeforester]

Summary

• Remove the legacy top-level basectl ci compatibility command and its dispatcher, help, completion, and test paths.
• Keep basectl setup --ci, basectl check --ci, and basectl doctor --ci as the canonical CI-safe interface.
• Update documentation, changelog, and AI command context with migration guidance.

Issue

Fixes #1756

Validation

• Focused help/completion BATS: 57 passed.
• Python suite from ./bin/base-test: 1055 passed, 1 skipped.
• bash -n, zsh -n, ShellCheck, and git diff --check passed for touched files.
• Full BATS run: alias-removal coverage passed; the run retains existing macOS diagnostic JSON and base-bash-libs fixture failures reproducible on the parent checkout.

Demo Impact

None.

Notes

The former alias was retained by the earlier CI-mode refactor; this issue removes that compatibility surface as a follow-up cleanup. CI-safe behavior and the BASE_CI/CI environment contracts are unchanged.

Checklist

• Branch name follows <category>/<issue>-<YYYYMMDD>-<slug> and matches the issue category.
• PR is scoped to one issue.
• PR body explains the changes and validation.
• Relevant tests pass or pre-existing failures are explained.
• Regression coverage is included for the removed command surface.
• Documentation is updated.
• AI context is updated.
• PR includes Fixes #1756.
• Demo Impact is meaningful or None.