fix: keep subagents in spawn order in the Agents panel

[adampeterhiggins]

Problem

The Agents panel is meant to list subagents in spawn order, but with a busy fleet the rows kept moving: each agent dropped to the bottom whenever it ran a tool call, and the elapsed timers reset to a few seconds.

Three things combined to cause it:

• The panel's order key (firstSeenAt) is the time of the oldest activity row the client holds for that agent.
• Thread snapshots keep only the newest 500 activities. In the thread where this showed up, every task.started row was 778–2,919 rows deep, so none of them reached the client.
• Each agent's task.progress row has a fixed id and is replaced on every tick. That keeps it in the window, but it makes it the agent's "first seen" row. Every update gave the agent a new firstSeenAt, so it re-sorted to the end. Its startedAt came from the same row, which is why the timers were wrong.

Fix

• Server: ingestion stamps the task's spawn time on its progress rows, both the activity row and the usage row, as spawnedAt. The time is cached per task when task.started arrives. On a cache miss (restart, TTL or eviction), a new repository query, getTaskSpawnedAt, reads the earliest start or stamped progress time from the database, which still has every row. If there is none, the first observed time is used and cached, so the value stays stable.
• Client: foldSubagentActivities sorts by spawnedAt when it is earlier than what it has seen. If a progress row is the first thing it sees for an agent, it also uses spawnedAt as that agent's startedAt. Web and mobile share this fold.

Workflow members sort on the same key, so they get the same fix.

Tests

• Fold ordering and start time when only replaced progress rows are retained (subagentRuntime.test.ts)
• Ingestion stamps the spawn time on the replaced progress row (ProviderRuntimeIngestion.test.ts)
• getTaskSpawnedAt reads it from a start row, from a stamped progress row, and returns none for an unknown task (ProjectionThreadActivities.test.ts)

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