feat(zsh): govern plugin standard reviews

[ss-o]

Summary

• establish the wiki Zsh Plugin Standard as the canonical public plugin-authoring guidance while keeping official Zsh documentation authoritative for shell semantics
• add required task/path routing, scoped instructions, and consistent plugin reviewer, README, scaffold, pattern, and bootstrap guidance
• define the maintainer-authorized semiannual wiki review procedure, including deterministic evidence, bounded issues: write, and idempotent UTC half-year issue creation

Related to #493.
Related to z-shell/wiki#840.

Verification

• python3 scripts/validate-agent-policy.py
• python3 -m unittest scripts/test_validate_agent_policy.py -v (72 tests)
• trunk check --no-fix --filter=-gitleaks on changed Markdown/instruction surfaces and the instruction workflow (no new issues)
• parsed .github/workflows/agent-instructions.yml with Ruby YAML
• scaffold source/unload regression coverage for manager-owned, plugin-owned, and repeated-source fpath state
• git diff --check

Agent handoff

• Status: Ready for review
• Current state: Public policy, routing, templates, patterns, validator contracts, and semiannual review governance are implemented and validated.
• Blockers: This PR depends on the coordinated z-shell/wiki PR for the canonical page update and the wiki-owned semiannual workflow tracked by docs: modernize the Zsh Plugin Standard and add semiannual review wiki#840. The wiki automation must land before or with policy that claims the review cycle is active.
• Next steps: Review this PR together with the coordinated wiki PR and private meta-workspace routing update; do not merge this PR ahead of unresolved coordinated dependencies.
• Related tracker/issues: feat(zsh): establish organization scripting standard #493, docs: modernize the Zsh Plugin Standard and add semiannual review wiki#840

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Pull request overview

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Suppressed comments (2)

scripts/test_validate_agent_policy.py:1484

• This test invokes zsh via subprocess.run(...) but does not handle environments where zsh is not installed. That makes the unit test suite fail with FileNotFoundError for local contributors (even though CI installs Zsh in the workflow). Add an explicit availability check and skip the test when Zsh is missing.

    def test_new_zsh_plugin_scaffold_owns_only_its_fpath_change(self) -> None:
        template = (
            PUBLIC_ROOT
            / ".github/skills/new-zsh-plugin/templates/plugin.plugin.zsh"
        ).read_text()

.github/skills/new-zsh-plugin/templates/plugin.plugin.zsh:10

• The comment says PMSPEC=f means the manager owns fpath setup, but the code correctly treats PMSPEC as a capability string (it checks for *f*). Update the comment to match the actual behavior so it doesn't mislead future readers.

# Optional manager capability: PMSPEC=f means the manager owns fpath setup.
# https://wiki.zshell.dev/community/zsh_plugin_standard#functions-directory