feat(wbfy): add generate-user-agent-instructions command

[exKAZUu]

Customer Summary

• wbfy generate-user-agent-instructions writes the organization's fixed user-level instructions for AI coding agents (Codex CLI, Claude Code, Gemini CLI) to the developer's home directory in one step.
• Every developer machine ends up with identical agent instructions covering the ghq clone convention, patience with long-running agents, and the no-AI-attribution rule.
• The command overwrites the existing files without merging, so any personal content in those files is replaced.

Technical Summary

• New packages/wbfy/src/generators/userAgentInstructions.ts writes ~/.codex/AGENTS.md, ~/.claude/CLAUDE.md, and ~/.gemini/GEMINI.md via fsUtil.generateFile and returns whether every file was written.
• packages/wbfy/src/index.ts registers the generate-user-agent-instructions subcommand, dispatched before any repository processing like the sibling apply-release-age-gate command, and sets a non-zero exit code when any file was skipped (e.g. a symlinked instruction file).
• The wbfy README documents the command.

Why

• User-level agent instructions are shared organization conventions rather than repository-specific ones, so they belong in a machine-level command instead of the per-repository generators.
• A skipped write must fail the command so scripts and developers notice that one agent still has stale instructions.

Testing

• HOME=<temp dir> bun src/index.ts generate-user-agent-instructions generates three identical files with the expected content and exits 0; a second run is idempotent.
• With ~/.claude/CLAUDE.md replaced by a symlink, the command skips that file with a warning, writes the other two, and exits 1.
• bun run verify passes.

[gemini-code-assist]

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