fix(subagents): preserve short ids across reloads

[tt-a1i]

Closes #63

What changed

• restore sa-N and btw-N high-water marks from the current Pi session branch on session_start
• append an invisible, versioned Session watermark after every successful spawn
• migrate existing sessions by recognizing legacy spawn/result entries when no watermark exists
• initialize both manager counters from the restored values

Why

The manager already avoided reuse during one extension runtime. The actual collision boundary was /reload: Pi retains the transcript while the OpenPI runtime and its in-memory counters restart at zero. A stale sa-1 prefix could then resolve to a different child. Keeping the high-water mark in Pi Session entries preserves Pi as the lifecycle source of truth and does not alter model context or tool schemas.

Validation

• bun run check
• bun run test — 822 Node tests + 30 Vitest tests, 0 failures
• focused ID/session tests — 26/26