Add just tasks serve composing heartbeat loop and plans server

Problem

Operators should not juggle three terminals (heartbeat, tasks server, optional manager). One recipe should start background heartbeat + tasks server with sane defaults and documented ports.

Context

• Heartbeat already runs detached after task-manager-start changes (background task-manager-run).
• This recipe may ensure heartbeat is running (e.g. call task-manager-ensure or document prerequisite) and start the server from add-tasks-serve-command-and-plans-server.
• Target UX: just tasks serve (alias acceptable during transition).
• Downstream assumptions from merged prerequisites:
• task-manager-heartbeat --dispatch-ready never mutates task frontmatter. Runtime state (started_at, claim) lives in the manager registry.
• Worktree conventions now default to .worktrees/<task-slug> rather than sibling directories.
• Snapshot status should surface escalation + PR-CI indicators for operator triage.

Possible Solutions

• A — Compose in justfile only: Shell orchestration; no new Python.
• B — Small Python launcher: Cross-platform process supervision; more moving parts.
• Recommended: A with clear logs and just tasks serve --help style comment block.

Plan

• [x] Add tasks serve recipe to justfile (and document conflict-free ports vs just plan serve / cloud).
• [x] Ensure idempotent behavior: second run attaches or no-ops with message (define explicitly).
• [x] Update AGENTS.md / task-manager skill with the one-liner operator path.

Implementation Progress

• Added tasks *ARGS recipe in Justfile:
• just tasks serve starts/ensures background heartbeat loop and then launches tasks server
• default tasks-server port 8005 (separate from just plans MkDocs at 8004)
• Idempotent heartbeat behavior:
• reuses existing tasks/logs/task_manager/task-manager-<owner>.heartbeat.pid when live
• stale pid file is cleaned up automatically
• second run prints heartbeat already running and only starts the server
• Follow-up robustness pass:
• recipe now accepts *ARGS parsing so just tasks serve owner=<owner> port=<port> interval=<seconds> works reliably
• heartbeat launch uses detached nohup + disown so Ctrl-C on server does not kill the background heartbeat
• just tasks now requires explicit serve action and defaults owner from TASK_MANAGER_OWNER or current user
• Updated operator docs:
• AGENTS.md one-liner path
• .codex/skills/task-manager/SKILL.md quick command list

QA Exploration

• N/A (operator tooling orchestration; validated with command smoke tests)
• [x] QA exploration completed (or N/A for non-UI tasks)

Review Feedback

• [x] Review cleared