docs(adr): propose runtime chroma knowledge

[ss-o]

Summary

• propose runtime-derived command knowledge as the canonical chroma source
• require argv-only asynchronous providers with cached frozen fallbacks
• define Git discovery, parser, drift-test, and external-command safety boundaries

Context

This records the architecture selected for F-Sy-H issue 73. The implementation is published separately in F-Sy-H so this ADR remains a reviewable organization-level decision.

Instruction impact review

1. Shared policy changes: the ADR adds canonical detail for chroma command knowledge. The manifest change registers that decision for deterministic inventory and routing.
2. Affected contexts: Codex, Claude Code, Copilot, Gemini CLI, and human maintainers when the task is architecture-decision or chroma-command-knowledge.
3. Canonical owner: decisions/0021-derive-chroma-knowledge-at-runtime.md in z-shell/.github.
4. Duplication or conflict: none identified. F-Sy-H owns the implementation; the ADR owns the cross-repository architectural decision.
5. Routing changes: .github/instruction-surfaces.json gains the required decision-0021 surface. The private workspace manifest and generated root composite are unaffected because no imported baseline or adapter content changes.
6. Delivery mechanism: supported runtimes select the required public manifest surface. No optional hook or skill is required.
7. Verification: public policy validators and their 84 and 99 test suites pass. Agent adapters and generated instruction sizes are unchanged, so runtime discovery mechanics are unaffected.

Validation

• git diff --check
• 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
• python3 -m unittest scripts/test_validate_zsh_standard_policy.py -v (99 tests)
• heading hierarchy and descriptive links reviewed against repository guidance
• status remains PROPOSED pending maintainer acceptance