feat(agent)!: remove FinOps heterogeneous-model guidance injection (ignores active provider)

[topcheer]

Summary

Removes the FinOps "Heterogeneous Model Selection" guidance module (sa-131), per a direct user directive: the injected guidance recommended Claude Opus / GPT-4 / Sonnet / GPT-4o-mini tiers unconditionally, completely ignoring the active provider (it fired on GLM-based sessions, where those model names are not even selectable). The text was appended to tool results ([FinOps Guidance: ...]), polluting result context with inapplicable advice.

Removed

• internal/agent/heterogeneous_model_guide.go + its test (the module: 10-call sliding-window exec-ratio heuristic + guidance text)
• docs/design/heterogeneous-model-guide.md (design doc for the removed behavior)
• 6 wiring points: Agent field, init, per-run reset ([business-logic] heterogeneousModel is the one detector the per-run reset block forgot (its reset() exists but is never called anywhere - the exact #677 pattern the block's own comment documents as previously fixed), so 'fires at most once per run' is really once per Agent lifetime and execRatio degrades into a session-long average; hub_package_guard sets initialized=true BEFORE doing any work and never clears it on failure, so a transient unreadable go.mod - or deterministically, any go.work-only monorepo root - permanently silences the guard for the whole session, while its 200ms deadline only constrains the parse loop (WalkDir is unbudgeted) letting partial fan-in maps get cached as authoritative #1466-A note), tool-loop injection call, compact-reset quota entry + its test fixture/assertion
• Release notes (v1.3.240 entry) left untouched as historical record

Verification

• Zero references remain (grep heterogeneousModel|HeterogeneousModel|hmCategory over internal/+cmd/ → empty)
• go test ./internal/agent/ full package ok 22.7s; go build -tags goolm ./... exit 0; gofmt clean

Co-Authored-By: ggcode noreply@ggcode.dev

[topcheer]

合并说明：techwriter_techwriter222_agent 代裁 approve（reviewer 作者回避；用户直令产品决策件；删除完整性已核——类型/字段/初始化/reset/注入/quota/测试/design doc 全部成对删除无半拆；release note 行为变化已登记）。CI 全绿。执行合并。