refactor(heartbeat): move task body renderers into bounded module

[huangruiteng]

Change

Q5 first heartbeat-prompt extraction slice: task-body and markdown renderers move from loopx/heartbeat_prompt.py into loopx/control_plane/heartbeat/task_body.py:

• render_heartbeat_task_body
• render_brief_heartbeat_task_body
• render_compact_heartbeat_task_body
• render_visible_goal_task_body
• render_traex_visible_goal_task_body
• _render_goal_task_body
• render_ark_managed_agent_goal_task_body
• render_thin_heartbeat_task_body
• heartbeat prompt markdown renderers

loopx/heartbeat_prompt.py keeps thin compatibility wrappers and drops from 1565 lines to 878 lines, satisfying the M6 heartbeat_prompt.py < 1200 gate.

Surfaces

• loopx/control_plane/heartbeat/task_body.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 4664d0b5c。这是对 PR #2979 的完整 owner review。该 PR 是 M6 Q5 的第一片真实替换：heartbeat task body 与 markdown renderers 迁入 loopx/control_plane/heartbeat/task_body.py，loopx/heartbeat_prompt.py 降到 878 行并保留薄兼容壳。

验证

• 全量 pytest：2306 passed, 2 skipped
• 聚焦 heartbeat/host/agent/import-boundary/maintainability：109 passed
• 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 task body renderers 现在位于 bounded heartbeat context，公共 facade 行为保持。按仓库 merge policy 路由合并。