Make gh issue assignee behavior configurable

[codeforester]

Problem

basectl gh issue create hardcodes --assignee codeforester for every created issue:

• cli/bash/commands/basectl/subcommands/gh.sh:42-45 documents that issues are assigned to codeforester.
• cli/bash/commands/basectl/subcommands/gh.sh:450-460 always passes --assignee codeforester.
• cli/bash/commands/basectl/tests/gh.bats:18 and cli/bash/commands/basectl/tests/gh.bats:117 test that hardcoded behavior.
• AGENTS.md:27 says Codex-created issues should be assigned to codeforester when GitHub allows it, which is a repo workflow preference, not a universal Base CLI default.

This is fragile for Base-managed repositories that are not owned by or connected to that assignee. In those repos, gh issue create --assignee codeforester can fail with an assignability error even though the issue itself would otherwise be valid.

Desired outcome

Keep Base's own workflow preference, but avoid making a single maintainer login part of the product's default issue creation behavior for every repository.

Possible direction

• Add an explicit --assignee <login> option.
• Add --no-assignee.
• Read a repo-local default from a Base/GitHub workflow config when present.
• For Base itself, preserve codeforester through repo-local guidance or config rather than hardcoded CLI behavior.

Acceptance criteria

• basectl gh issue create can create an issue without forcing a hardcoded assignee.
• Base's own repository workflow can still assign codeforester when desired.
• Help text explains the default assignee behavior accurately.
• Tests cover default behavior, explicit assignee, no-assignee, and current Base repo expectations.