fix: harden dynamic tool handlers against deadlock, hangs, and runaway output

[electronicBlacksmith]

What Changed

• Added drainProcessWithLimits() that drains stdout/stderr concurrently with a hard timeout (SIGTERM, then SIGKILL after 2s grace period)
• Added readStreamWithCap() that caps output at 1MB but keeps reading past the cap so the child process never blocks on a full pipe
• Configurable via PHANTOM_DYNAMIC_HANDLER_TIMEOUT_MS (default 60s) and PHANTOM_DYNAMIC_HANDLER_MAX_OUTPUT_BYTES (default 1MB)
• Timeout returns error message with partial output snippet

Why

Dynamic MCP tool handlers (shell/script) can hang indefinitely or produce unbounded output. A runaway find / or infinite loop blocks the agent forever. If both stdout and stderr fill their OS pipe buffers (~64KB each), the child deadlocks waiting for a reader while the parent blocks reading one pipe.

How I Tested

• bun test src/mcp/__tests__/dynamic-handlers.test.ts - 25 pass (timeout, output cap, concurrent drain, graceful shutdown)
• bun test src/mcp/__tests__/dynamic-tools.test.ts - 8 pass
• bun run lint - clean
• bun run typecheck - clean
• Full suite: 976 pass

Checklist

• Tests pass (bun test)
• Lint passes (bun run lint)
• Typecheck passes (bun run typecheck)
• No secrets or .env files included
• Files stay under 300 lines
• No Cardinal Rule violations
• No default exports or barrel files added