Per-session resource usage: CPU/memory per session, worktree disk, transcript store size

[xerhab]

Context

The heartbeat reports container-level memory only (memory_usage() reads the cgroup in agent/hub-agent.py), and the compose file sizes mem_limit/cpus/pids_limit against MAX_SESSIONS. Nothing attributes usage to individual sessions, and disk isn't tracked at all — worktrees under REPOS_ROOT/.agenthub/worktrees and transcripts under ~/.claude/projects both grow unbounded.

Problem

• When a host container approaches its memory limit, there's no way to see which session is responsible — a runaway build or a huge repo checkout looks the same as six healthy sessions. The only remedy today is guessing or restarting the whole container.
• Worktrees (with node_modules, build artifacts, etc.) can be large; killed sessions keep their branch but freed worktrees, while stopped-but-never-deleted sessions keep worktrees on disk indefinitely. Transcript JSONLs also accumulate forever and are re-parsed by usage_report() — parse cost grows with history.

Proposal

• Per-session CPU/RSS: each session is a tmux session whose pane PID is queryable (tmux list-panes -t agent-<id> -F '#{pane_pid}'); walk its process subtree via /proc/<pid>/.. (stdlib-only, matching the repo's no-pip constraint) and sum RSS + utime/stime deltas between beats. Report {rssBytes, cpuPct} in _session_payload().
• Disk: du-equivalent of the worktree (cheap incremental: full scan on the slow USAGE_EVERY cadence, like usage parsing) plus the transcript directory size.
• UI: add the numbers to the session card, and turn the host memory line into used/limit with a top-consumer hint. Sort options by RSS (pairs with the dashboard filtering issue).
• Alert hook: host memory > N% of limit is a natural ntfy alert (edge-triggered like the others in heartbeatAlerts()), now actionable because the payload says which session to kill.

Touchpoints

• agent/hub-agent.py — proc-tree sampling, _session_payload(), slow-cadence disk scan
• agent-hub/server.js — optional memory alert
• agent-hub/public/index.html — display