fix(shell): stop sourced shell files from enabling errexit

[Dreamcoder08]

Summary

Snippets sourced by .zshrc/.bashrc started with set -euo pipefail, so the first failing command closed an interactive terminal.

Changes

• LS_COLORS, fzf and bat snippet generators no longer emit shell options (regenerated variants included)
• .zshrc drops errexit/nounset and resolves the mode like fish
• dreamcoder-icons.sh no longer enables errexit; a bats test sources every shell lib and asserts no option changed

Verification

tests/test_sourced_snippets_shell_options.py and tests/shell/test_sourced_shell_lib.bats (RED before, GREEN after); zsh survives false after startup.

Chain Context

Field	Value
Chain	ML4W 2.16 + Herdr 0.9.1 + Light/Dark-only stabilisation
Tracker PR	Not needed (stacked PRs to main)
Position	4 of 6
Base	pr/03-theme-mode-and-listener
Depends on	#13
Follow-up	#15
Review budget	80 / 400
Starts at	the tip of pr/03-theme-mode-and-listener
Ends with	fix(shell): stop sourced shell files from enabling errexit

Chain Overview

main
 └── #11 fix(ml4w)
  └── #12 fix(install)
   └── #13 fix(theme)
    └── 📍 #14 fix(shell)
     └── #15 refactor(theme)
      └── #16 chore(quality)

Scope

• Includes: see Changes above
• Excludes:
• None

Autonomy

• CI is expected to pass for this PR branch
• This PR has one deliverable scope
• This PR can be rolled back without unrelated changes
• Tests, docs, or manual verification cover this unit

[Dreamcoder08]

Verified independently at this slice tip (stacked PRs do not run CI until retargeted to main): pytest exit 0, ruff clean, bats 1 failure. The single bats failure is apply-theme-mode.sh kanagawa bridge carries night-derived colors, which already fails on main; it is removed together with Night in #15 (bats 0 failures there).