fix: refuse stale-answer replay in OpenCode and Pi

[antauth-alfonso]

Summary

Refuse meaningful trailing assistant history instead of silently deleting it and replaying the previous question.

• Share one Core guard between OpenCode and Pi. Remove only provably empty trailers; reject meaningful text, tools, thinking, and unknown content before signing or model dispatch.
• Return a terminal local OpenCode 400. Private response provenance prevents account fallback even when upstream 400 responses are configured for fallback; genuine provider responses retain that policy.
• Validate Pi history before and after conversion so lowering cannot hide meaningful content.
• Bind the mock relay explicitly to IPv4 loopback and verify exclusive endpoint ownership.

Verification

• Core: 325 tests; OpenCode: 1,726 tests; Pi: 155 tests.
• Isolated process E2E: 52/52, including a real OpenCode child proving no extra model request and a non-retryable local error.
• Independent mutations fail when the history guard, Pi pre-conversion guard, local-response provenance, or mock endpoint ownership is removed.
• Workspace typechecks, packed Node CLI/TUI startup, real Pi tool round-trip, emitted-import validation, and workspace lock checks pass.

Draft for clean-runner CI. No live host placement, credential migration, main integration, or package release is included. This guard prevents replay; it does not repair the host's non-atomic message/parts persistence.

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Summary by cubic

Refuses meaningful trailing assistant history in OpenCode and Pi instead of silently discarding it and replaying the previous question.

• A shared Core guard removes only provably empty trailers and rejects text, tools, thinking, and unknown content before billing, caching, signing, or model dispatch.
• OpenCode returns a terminal local 400 with private response provenance, so configured upstream 400 fallback policies cannot route the request to another account; genuine provider 400s keep their existing policy.
• Pi validates history before and after message conversion so lowering cannot hide meaningful content.
• The mock relay now binds exclusively to IPv4 loopback, with an endpoint-ownership test covering the failure mode.
• Core, OpenCode, Pi, and isolated end-to-end tests pass, including verification that refused requests make no model request and do not enter retry state.

^{Written for commit 9e8419a. Summary will update on new commits.}