Block-fingerprint readback for injected clients (replace byte-compare)

[NOirBRight]

This was generated by AI during triage.

Summary

Replace byte-equality round-trip readback (verify_apply_readback, gateway.rs:1146-1303) with block-fingerprint readback for clients under Provider Injection.

Problem

Today readback re-serializes the expected config and byte-compares against the whole file. Under coexistence semantics the user may add/edit their own providers at any time — any such edit currently reads as drift. Byte-compare is fundamentally incompatible with Provider Injection.

Design (decided)

• Readback checks only the Injected Block: presence, fingerprint match (endpoint, protocol, projected model set, credential reference — never the credential value).
• Foreign content is never validated and never reported as drift.
• The client's global default-model key (e.g. DSH agent-default-model) is read and surfaced as informational state ("active / pointing elsewhere"), never as an error — injection never owns it.
• Concurrent-edit safety applies only to the injected block and the single credential key; writes remain backup + atomic.

Acceptance criteria

• Injected client passes readback after the user adds/removes unrelated providers
• Tampered or deleted injected block reports truthful drift with actionable message
• Credential values never appear in readback evidence
• Legacy takeover clients keep byte-compare until their migration phase

Related

• Add DeepSeek Harness (DSH) as CodexHub's fifth fully supported managed client #430, ADR-0004

[NOirBRight]

Track 0 note from #441: the block-fingerprint implementation landed on the 0.1.9-beta.1 line (4ed8c1a). This ticket stays open until a dedicated acceptance pass against current main confirms the issue body checkboxes. Not a blocker for #413 / #414.