fix(proxy): fall through to OPA on identity resolution failure

[pdettori]

Summary

• Fix sandbox egress 403 when --policy explicitly allows an endpoint but no binaries are specified
• The CONNECT proxy now falls through to OPA evaluation with a sentinel identity instead of hard-denying on /proc/net/tcp lookup failure
• Empty binaries in a network policy rule is treated as "any binary allowed" (the endpoint itself is the gate)

Root Cause

Two interacting bugs:

1. Identity resolution fails in K8s — peer_addr.port() returns ports outside the ephemeral range (4337, 5204, 5952) due to namespace boundary timing in the privileged container + veth setup. The /proc/<pid>/net/tcp lookup never finds them.

2. Empty binaries = unconditional deny — The Rego binary_allowed rule uses some b := policy.binaries[_] which yields nothing on an empty array, so even successful identity resolution would be denied.

Changes

File	Change
sandbox-policy.rego	Add binary_allowed(policy, _) rule for empty binaries
proxy.rs	Replace hard-deny with sentinel <unresolved> identity fallback
opa.rs	Add 3 unit tests
testdata/sandbox-policy.yaml	Add test policy with no binaries

Security

• <unresolved> contains angle brackets — invalid filesystem path, can never match a real binary entry
• Policies with explicit binaries restrictions remain enforced (the sentinel won't match)
• Only policies that intentionally omit binary restrictions benefit from the fallback

Test plan

• cargo test -p openshell-sandbox passes (new + existing tests)
• Deploy to Kind, openshell sandbox create --policy egress-allow.yaml, verify curl https://github.com works inside
• Verify policy WITH explicit binaries still blocks unrecognized binaries

Assisted-By: Claude Code

[pdettori]

Self-review notes:

Good fix for the identity-resolution + empty-binaries interaction. Rego logic is correct, sentinel is safe, tests cover the security invariant.

Blocking issue — CI clippy failure:

error: fields `binary`, `binary_pid`, and `ancestors` are never read
  --> crates/openshell-sandbox/src/proxy.rs:1219:5

The IdentityError struct (line 1217) has fields that are written but never read after this change. Fix options:

1. Include these fields in the warn!() message so they provide diagnostics (preferred — helps debug identity resolution in prod)
2. Prefix with _ if they're for future use

Once clippy passes, this LGTM.