Make gh issue assignee configurable

[codeforester]

Summary

• Add --assignee <login> and --no-assignee to basectl gh issue create.
• Stop forcing a product-wide hardcoded assignee; read project.issue_defaults.assignee from .github/base-project.yml when present.
• Preserve Base repo assignment through repo-local config and update docs, completions, guidance, and AI context.
• Allow assignee in the Project config parser while keeping it out of Project field updates.

Validation

• env BASE_BASH_LIBS_DIR=/Users/rameshhp/work/base-bash-libs/lib/bash bats cli/bash/commands/basectl/tests/gh.bats
• env BASE_BASH_LIBS_DIR=/Users/rameshhp/work/base-bash-libs/lib/bash bats cli/bash/commands/basectl/tests/completions.bats
• PYTHONPATH=lib/python:cli/python /Users/rameshhp/.base.d/base/.venv/bin/python -m pytest cli/python/base_github_projects/tests/test_engine.py -q
• env -u BASE_HOME BASE_BASH_LIBS_DIR=/Users/rameshhp/work/base-bash-libs/lib/bash ./bin/base-test
• git diff --check

Fixes #894