docs: proposed plan for cpex-openshell integration

[imolloy]

Summary

Adds a draft proposal (docs/plans/2026-06-17-001-feat-cpex-openshell-integration.md) exploring whether CPEX can provide an optional, operator-owned identity and fine-grained authorization layer for OpenShell's L7 egress proxy. This is a solicitation for feedback — not an implementation, and not a drop-in replacement for the existing OPA/regorus policy path.

The proposal frames a Phase 0 compatibility and threat-model spike as a decision gate before any in-tree product integration. It documents the current OpenShell and CPEX integration context (including hard blockers like the Rust 1.96 vs. 1.95.0 MSRV mismatch), sets design constraints, and sketches a phased delivery plan.

What's in the doc

• Invariants — OpenShell retains L4/SSRF/process/canonicalization/credential-protection; CPEX cannot widen the baseline; deny always wins; identity must be authenticated with provenance; failure is closed; no secret disclosure.
• Initial scope — narrow CPEX authorization adapter for REST request admission, one local PDP (Cedar or CEL after spike), operator-owned digest-pinned bundle. Explicitly excludes JWT/OAuth exchange, response mutation, PII, taint/session state, Valkey, and non-REST protocols in v1.
• Configuration ownership — gateway-owned bundle registry (preferred) or typed SandboxPolicy protobuf reference; no inline APL in agent-editable policy.
• Evaluation order, minimal L7Authorizer interface, EffectiveL7Policy snapshot semantics for atomic reload.
• Phased plan — Phase 0 spike → Phase 1 RFC → Phase 2 refactor → Phase 3 feature-gated shadow-then-enforce pilot → Phase 4 separately approved follow-ups.
• Validation matrix, non-goals, and requested feedback — with explicit questions on ownership model, identity source, PDP choice, and pilot SLOs.

Non-goals for this PR

• No code changes, no schema changes, no dependency additions.
• No commitment to adopt CPEX — Phase 0 is a decision gate, and rejection is a valid outcome.

[araujof]

Nice job! LGTM