agentop timeline shows workflow subagents as 'subagent <id>' #1756

Problem

When Claude Code's Workflow tool starts subagents, the agentop timeline pane labels each one subagent <id>. Subagents that the Agent tool starts show their description, for example "Implement Task 3: SSE and decode".

Seen on session f3cff1d8-c329-4b45-b03e-f6041bec8187 (25 workflow agents, all unlabeled).

Cause

1. The timeline takes a subagent's label from the Agent tool call on the wire (core/session/timeline/agents.go, the spawn.desc match). A workflow starts its agents inside Claude Code, so no such call goes over the wire.
2. Claude Code still writes each label to disk, in <session>/subagents/workflows/<run>/agent-<id>.meta.json ("description": "merge T2"). readSubagents in core/observe/claude/harvest.go reads only <session>/subagents/agent-*.meta.json, one level up.
3. Only the events pane uses harvested labels (cmd/agentop/tui/events_blocks.go subagentLabel). The timeline pane does not.
4. The incremental harvest skips a session when its main transcript did not change. During a workflow run, the main transcript stays almost unchanged, so new labels do not arrive while the run is live.

Fix

1. readSubagents also reads subagents/workflows/*/agent-*.meta.json.
2. An incremental harvest that skips a transcript still re-reads that session's meta files. They are small.
3. The timeline pane uses the harvested description for a subagent that has no spawn call on the wire.
4. The timeline pane asks for a harvest when it shows a subagent with no label, at most once per 15 s.