docs(design): add low-compute router-only candidate

[wefio]

变更描述

What（改了什么）：Add the pre-existing local convergence design draft, which was absent from upstream main, with a new §3.12 low-compute router-only candidate and a narrow §8 correction distinguishing passive shadow logging from executed causal experiments.

Why（为什么改 / 解决什么问题）：Record the user's agreed direction: reuse inexpensive gating architecture, not pretrained MoE weights or neural experts. Preserve the earlier draft rather than silently discard it. Earlier theoretical claims remain draft hypotheses, not verified findings; §3.12 explicitly narrows the candidate's evidence and budget assumptions.

Changes（关键改动点，按文件或模块列出）：

• Sole file: docs/design/agent-convergence-feedback-design.md (633-line addition because the original local draft was never included upstream).
• 32→4 linear baseline: 132 parameters; optional 32→16→4 sigmoid MLP: 596 parameters.
• Fixed none/cue/resurface/retrieve executors; external permission and hard-budget gates.
• Observed-action regression, scoped reward evidence, real randomized interventions, and numerical validation prerequisites.
• No runtime code, training, live benchmarks, enabled policies, or default changes.

完成检查项

本地质量检查

• Applicable local documentation checks passed; full npm run verify:static was not run locally, but Static CI passed (observed through RCP).
• Scoped npm run agent:verify -- docs/design/agent-convergence-feedback-design.md passed.
• No dependency/lockfile changes; isolated npm ci reported zero vulnerabilities.
• No subpackage changes.
• npm run docs:check passed: 111 docs, 0 errors, one pre-existing filename warning for topology-namesakes-evaluation.md; doc-maintenance followed.
• No new code methods or complexity changes.
• No generated artifacts committed.

RCP（Repository Control Plane）

• In-flight board goal registered before editing: 1788585556951_000015, agent router-docs; resolved after documentation delivery and verification.
• Scoped agent:verify passed and .nmg/verification/latest.json records evidence. No existing Contract covers this document, so no unrelated Contract was bound or declared reconciled.
• RCP forge-status --pr 23 --json observed all seven checks COMPLETED/SUCCESS for head 62f820c4fcc6b5f7ecdd967d8de01f0dd160ff44.
• Only owned documentation committed; original checkout's ongoing cherry-pick and unrelated local commits preserved using an isolated worktree based on upstream main.

CI 完成确认

• RCP observation: All checks passed successful.
• CodeFactor passed.
• Static job passed.

Applicable checks have passed; readiness and merge remain explicit forge operations. User explicitly authorized PR merge, subject to repository permissions and protection; no bypass requested.