Add GitHub issue readiness check

[codeforester]

Summary

• add basectl gh issue readiness <number> for read-only implementation issue readiness checks
• validate required issue body sections and optional Base Project fields via explicit Project owner/number flags
• update Bash/Zsh completions and GitHub workflow docs for the agent-assignment gate

Fixes #1560

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
• env BASE_BASH_LIBS_DIR=/Users/rameshhp/work/base-bash-libs/lib/bash bats lib/shell/completions/tests/completions.bats
• bash -n cli/bash/commands/basectl/subcommands/gh.sh lib/shell/completions/basectl_completion.sh lib/shell/completions/basectl_completion.zsh
• shellcheck -x cli/bash/commands/basectl/subcommands/gh.sh lib/shell/completions/basectl_completion.sh
• zsh -n lib/shell/completions/basectl_completion.zsh
• git diff --check
• live smoke: ./bin/basectl gh issue readiness 1560 --repo basefoundry/base --project-owner basefoundry --project-number 10