refactor(heartbeat): move heartbeat prompt builder into bounded module

[huangruiteng]

Change

Q5 second heartbeat-prompt extraction slice: build_heartbeat_prompt and build_heartbeat_prompt_error_payload move from loopx/heartbeat_prompt.py into loopx/control_plane/heartbeat/builder.py.

loopx/heartbeat_prompt.py keeps thin compatibility wrappers and drops from 878 lines to 481 lines.

Surfaces

• loopx/control_plane/heartbeat/builder.py
• loopx/heartbeat_prompt.py

Validation

• Full pytest: 2306 passed, 2 skipped.
• Focused heartbeat/host/agent/import-boundary/maintainability: 109 passed.
• Ruff: All checks passed.
• loopx canary premerge --from-git-diff: passed, selected=15 failures=0, self_merge_allowed=true.

No failures or skips beyond the pre-existing suite skips. No manual holds.

Routing

Route: codex-side-bypass; not assigned to codex-quality-qualification.

[huangruiteng]

Review: APPROVED

head 8b2fb0f6b。这是对 PR #2980 的完整 owner review。该 PR 是 M6 Q5 的第二片真实替换：heartbeat prompt builder 迁入 loopx/control_plane/heartbeat/builder.py，loopx/heartbeat_prompt.py 降到 481 行并保留薄兼容壳；同时将 task body renderer 选择抽成 _select_task_body_renderer，让迁移后的函数回到维护性棘轮阈值内（90 statements / 60 decision points）。

验证

• 聚焦 heartbeat/host/import-boundary/maintainability：138 passed
• maintainability ratchet：clean（无 oversized_decision_function）
• ruff：通过
• loopx canary premerge --from-git-diff：selected=15 failures=0，self_merge_allowed=true
• GitHub checks：dependency-review / pytest / build 全部通过

结论

无 blocker。heartbeat prompt builder 现在位于 bounded heartbeat context，公共 facade 行为保持。按仓库 merge policy 路由合并。