fix(zsh): resolve zsh-lint default-assignment warnings

[ss-o]

Problem

The strict zsh-lint rollout-readiness corpus reports 11 quoting/unquoted-var warnings in public/zsh/init.zsh at the top-level default assignments for ZI configuration and HISTFILE. The readiness gate permits no suppressions or waivers.

Required work

• verify each diagnostic against the sourced-library execution profile and native Zsh behavior;
• preserve empty values, spaces, and caller option states such as SH_WORD_SPLIT;
• add focused regression coverage before changing production source;
• make the smallest source correction that clears all 11 warnings;
• run native syntax and the repository installer/loader test suite.

Acceptance criteria

All 11 warnings are removed without suppressions, existing installer stream substitution remains correct, native tests pass, and the shared 19-file corpus reports no warning from src.

[ss-o]

Local implementation evidence on feature-180-zsh-lint-warnings: all default-assignment expansions in public/zsh/init.zsh are quoted, install.sh stream substitution is preserved, and public/checksum.txt is regenerated. A new SH_WORD_SPLIT regression proves each colon command receives one argument while configured and derived values containing spaces remain intact. sh ./tests/installers.sh passes and zsh-lint reports zero diagnostics for init.zsh. Changes remain uncommitted and unpublished.

[ss-o]

Resolved by #181, merged into next on 2026-08-16.

This issue remained open because GitHub only closes issues automatically when a pull request merges into the default branch (main). The Closes #180 keyword in a next-targeted pull request therefore has no effect. Closing manually per runbooks/triage.md Step 6.

The work is on next. Promotion to main is tracked separately.