fix(streaming): bound subprocess memory

[drewstone]

What

• remove per-event progress timers and repeated reactions retained for the lifetime of a subprocess
• retain subprocess diagnostics in a fixed 64 KiB head-and-tail buffer across every CLI backend
• keep model output, token budgets, time budgets, and replay behavior unchanged
• ignore request-private Pi directories left by hard process exits

Measured evidence

• failing Discovery run: the bridge reached Node's approximately 4.1 GB heap limit and exited
• before: 200,000 synthetic Pi events peaked at 525,572 KiB RSS
• after: 1,600,000 events completed in 1.46 s, peaked at 305,520 KiB RSS, and settled at 41.3 MB heap
• DeepSeek V4 Flash reviewed the full diff through Pi; its two low-severity findings were fixed before commit

Checks

• pnpm test: 584 passed, 6 skipped
• pnpm typecheck
• git diff --check origin/main...HEAD
• git merge-tree --write-tree origin/main HEAD