Prevent Zsh special parameter name collisions in scoped guidance

[ss-o]

Summary

The canonical Zsh scripting guidance does not explicitly prevent unrelated
scratch variables from reusing shell-owned special parameter names. That can
turn an ordinary local declaration into a runtime failure or a change to shell
state.

The released Zsh manual identifies status as the exit-status parameter and
path as the array tied to PATH. A clean-shell probe confirms that assigning
to a local status fails because it is read-only, while localizing path
changes command lookup through PATH.

Proposed change

• Add one required scoped rule covering all five execution profiles.
• Require purpose-specific names instead of repurposing Zsh special parameters
  for unrelated local or scratch data.
• Keep the machine-readable rule catalog, validator inventories, startup
  membership, and deterministic tests synchronized.
• Preserve the existing instruction route and canonical owner.

Instruction impact review

1. Classification: scoped normative Zsh guidance plus its existing
   machine-readable enforcement metadata.
2. Consumers and contexts: Codex, Claude Code, Copilot, Gemini CLI, and
   human contributors working with enrolled Zsh source in standalone,
   startup-file, sourced-library, autoload-function, and test-fixture profiles.
3. Canonical owner: .github/instructions/zsh-scripting.instructions.md
   remains the correct prose owner; lib/zsh-standard-policy.json remains the
   metadata owner.
4. Duplication or contradiction: none found. Existing parameter rules cover
   explicit scope and dynamic scope, but not collisions with special parameter
   names.
5. Manifest routing: no added, changed, or removed route is needed. The
   existing instruction-zsh-scripting surface already reaches the affected
   paths and consumers.
6. Runtime delivery: yes. Every supported runtime receives the mandatory
   scoped instruction without relying on an optional hook or skill.
7. Generated output and limits: the public agent-policy and Zsh-policy
   validators pass. The private generated root composite is unaffected because
   this does not change the imported public baseline or private overlay.

Acceptance criteria

• Canonical prose explains the status and path failure modes.
• Policy metadata and exact validator inventories contain the new rule.
• Startup-profile membership remains synchronized.
• A focused test protects the failure-mode wording.
• Public agent-policy and Zsh-policy tests and validators pass.

References

• Zsh Parameters manual
• runbooks/instruction-update.md
• decisions/0015-zsh-scripting-standard.md