Add opt-in Git remote reachability diagnostics

[codeforester]

Summary

• Add --remote-network to project basectl check and basectl doctor flows.
• Report bounded git ls-remote reachability as BASE-P083 with sanitized JSON details.
• Keep default project Git diagnostics local-only, including pre-venv fallback paths.

Validation

• env -u BASE_HOME PYTHONPATH=/Users/rameshhp/work/base-worktrees/feature-527-remote-reachability/lib/python:/Users/rameshhp/work/base-worktrees/feature-527-remote-reachability/cli/python /Users/rameshhp/.base.d/base/.venv/bin/python -m pytest cli/python/base_setup/tests/test_git_remote.py -q
• env -u BASE_HOME bats cli/bash/commands/basectl/tests/check.bats cli/bash/commands/basectl/tests/doctor.bats
• env -u BASE_HOME ./bin/base-test
• git diff --check
• git diff --cached --check

Demo Impact

• None. Diagnostic CLI behavior only.

Closes #527