feat: polish bashrc stack helpers

[julsemaan]

Summary

• harden AI commit message generation so tool failures/rate limits do not become commit messages
• make gcoto helpers proper functions and add staged-change/suspicious-output guardrails
• add idempotent PATH prepending and more robust optional completion/fzf behavior
• expand bashrc smoke coverage for the new helper paths

Testing

• bash -n profile/.bashrc_append profile/bashrc.d/*.bash tests/bashrc-smoke.sh tests/lib/assert.sh
• bash tests/bashrc-smoke.sh