docs: add native router and issue-agent RFCs

[KooshaPari]

Summary

• add a native-router workstream RFC for feat(backend): unified RouterBackend contract across CLIProxyAPI, Bifrost, VibeProxy, and native hot paths #5670 that sequences the deferred feat(router-eval): add retained optimization gate #6071/[codex] Generalize backend failure cooldown state #6079/[codex] Document native router backend migration plan #6081/feat(sidecar): add provider manifest client #6083 slices
• add an issue-agent security-boundary RFC for feat(backend): AI-assisted bug report, triage, fix, and PR workflow #5620 before reviving implementation work
• document staged acceptance gates, non-goals, and source-grounded anchors for both workstreams

Validation

• npm run check:fabricated-docs
• npm run check:doc-links

Refs #5670
Refs #5620

[gemini-code-assist]

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[diegosouzapw]

🔀 Cycle moved: base retargeted to release/v3.8.46 — release/v3.8.45 is frozen for the release in progress (#6313). No action needed.

[diegosouzapw]

Thanks for this — closing this draft to keep the queue focused, but the idea is not lost.

This repo tracks the sidecar/native-router workstream centrally under #6044 (and the broader routing unification under #5670), and the previous grouped review already flagged these drafts as pieces of that not-yet-agreed subsystem. Landing pieces individually before the design is settled would commit the core to a direction ahead of that agreement.

The idea from this PR is now captured in #5670 / #6044 (design non-goals salvaged there), credited to you ("Idea from #6159 by @KooshaPari"), so it can be picked up cleanly once the workstream design lands. Feel free to comment there with any additional context from this branch — or reopen with a narrower, rebased diff anytime if you'd like to take it further yourself.

Thanks again for the contribution!