Docs: create ADR documenting cargo-audit quality-gate adoption

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Summary

Raised via: PR #323, requested by @leynos

docs/developers-guide.md documents the cargo-audit tooling and make audit workflow, but no Architecture Decision Record (ADR) exists to capture the formal rationale for adopting cargo-audit as a quality gate.

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Context

PR #323 integrates cargo-audit as a Makefile and CI gate. The developer guide covers the mechanics, but the decision record is absent. Existing ADRs (adr-001, adr-002, adr-003) establish precedent for the format.

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Resolution

Create docs/adr-NNN-adopt-cargo-audit-quality-gate.md following the repository ADR template, covering:

• Context: RustSec advisory database, prior absence of automated dependency auditing.
• Decision: Adopt cargo-audit (via cargo binstall) as a Makefile target (make audit) and CI gate.
• Consequences: Advisory database fetched on each CI run; Cargo.lock required adjacent to each audited manifest; allowed warnings vs. hard failures under default policy.
• Migration plan: Remove stale lockfiles, refresh Cargo.lock as part of adoption.
• Goals / Non-goals: Gate on vulnerabilities; informational advisories remain warnings.

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References

• PR: Adopt cargo audit gate #323
• Existing ADRs: docs/adr-001-login-auth-reply-compatibility-layers.md, docs/adr-002-compatibility-guardrails-and-augmentation.md, docs/adr-003-login-authentication-and-reply-augmentation.md

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🔗 Related PRs

leynos/prosidy-darn#9 - Create baseline developer docs and ADR locations (1.0.1) [merged]
leynos/corbusier#79 - Add joint Rust and node audit gate [merged]
leynos/episodic#101 - Define source-to-script REST vertical slice [merged]
leynos/stilyagi#13 - Import stricter Python lint tiers [merged]
leynos/dear-diary#28 - Add Whitaker to lint checks [merged]

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