docs(project): define sub-issue hierarchy policy

[ss-o]

Summary

• add the canonical parent issue, sub-issue, and dependency runbook
• add the parent-only meta:initiative label and cross-repository initiative form
• route the runbook to all supported consumers and link it from existing policy
• add a focused repository policy contract test

Instruction impact review

1. Classification: Shared organization policy with canonical-detail runbook guidance and an issue-form entry point.
2. Consumers: Codex, Claude Code, Copilot, Gemini CLI, humans, and public Z-shell repositories consuming the organization baseline.
3. Canonical owner: z-shell/.github remains the canonical owner.
4. Duplication: runbooks/sub-issues.md owns hierarchy rules; tracker, triage, label, and agent surfaces link to it instead of copying the procedure.
5. Manifest routing: The public manifest adds runbook-sub-issues for project-tracking and sub-issue-management. Private composite regeneration remains a later gitlink reconciliation step after this public change lands.
6. Runtime delivery: AGENTS.md links the mandatory runbook, so delivery does not depend on an optional hook, agent, or skill.
7. Validation: Public validators and unit tests pass; AGENTS.md is 11,326 bytes, below the 32,768-byte limit. Manual runtime discovery was not performed.

Verification

• python3 -m unittest scripts/test_validate_agent_policy.py -v (84 passed)
• python3 -m unittest scripts/test_validate_zsh_standard_policy.py -v (99 passed)
• python3 scripts/validate-zsh-standard-policy.py
• python3 scripts/validate-agent-policy.py
• sh scripts/test-labels-dry-run.sh
• ruby scripts/test-labeler-config-audit.rb
• issue-form and label YAML parsing, canonical-label cross-check
• read-only label preview for z-shell/.github and z-shell/zsh-lint
• git diff --check

Closes #521