fix(agent): deduplicate instruction loading and gate writes

[ss-o]

Summary

• Treat byte-exact instruction content already present in an active context as loaded.
• Require explicit external-write authority for issues, comments, pull requests, and tracker records.
• Add contract coverage for both policy guarantees.

Closes #532

Instruction impact review

1. Classification: shared organization policy, handoff procedure, and executable contract assertions.
2. Consumers: Codex, Claude Code, Copilot, Gemini CLI, humans, and repositories importing the public baseline.
3. Canonical ownership: AGENTS.md remains the shared owner; .github/AGENT_MEMORY.md remains the handoff owner.
4. Duplication: no new policy owner is introduced. The associated zsh-eza change removes duplicated adapter prose.
5. Routing: the public manifest needs no route change. The private route guard is handled in the separate private integration.
6. Delivery: mandatory rules remain available through public AGENTS.md without an optional hook or skill.
7. Validation: the public validator and 75 tests pass. The generated private composite is validated separately and remains below its size limit. Manual runtime discovery is unverified.

Verification

• python3 scripts/validate-agent-policy.py --root .
• python3 -m unittest discover -s scripts -p test_validate_agent_policy.py -q
• git diff --check

Agent handoff

Status: Ready for review
Repository: z-shell/.github
Branch/PR: bug-532
Tracker/Issue: #532

Current state

• The policy, memory protocol, and contract tests are committed and published.

Verification

• Public validator: passed.
• Public unit suite: 75 tests passed.
• Diff check: passed.

Blockers

• None.

Next steps

1. Review and merge this PR.
2. Pin the merged public commit in the private meta-workspace and regenerate its composite.