docs(testing): isolate autoload paths from other checkouts

[ss-o]

Summary

• Require Zsh tests to control FPATH and fpath from the checkout under test.
• Prevent another worktree or installation from silently supplying a same-named autoload.

Evidence

While validating z-shell/F-Sy-H#77 in z-shell/F-Sy-H#134, ZUnit initially executed _fsh_make_targets from a stale primary checkout inherited through FPATH. Clearing FPATH made the regression exercise the intended worktree and pass.

Instruction impact review

1. Classification: scoped testing guidance.
2. Consumers: Codex, Claude Code, Copilot, Gemini CLI, and humans in repositories testing Zsh autoloads.
3. Canonical owner: .github/instructions/testing.instructions.md remains the correct owner.
4. Duplication and contradiction: none found. Existing Zsh guidance controls trusted paths but does not prevent cross-checkout test selection.
5. Routing: no manifest change. instruction-testing already routes this file for testing tasks.
6. Runtime delivery: all supported runtimes receive the required scoped instruction through the manifest, with no optional hook or skill dependency.
7. Generated output: no adapter or generated composite changes. AGENTS.md and its size limit are unaffected.

Verification

• python3 scripts/validate-agent-policy.py
• python3 -m unittest scripts/test_validate_agent_policy.py -v (84 tests)
• python3 scripts/validate-zsh-standard-policy.py
• trunk check --no-progress --no-fix .github/instructions/testing.instructions.md
• git diff --check

Agent handoff

No handoff needed.