pi: move subagent system prompts out of argv for the Windows command-line cap

Pi subagents (historian/dreamer/sidekick) spawn a pi subprocess and passed
the system prompt (~60KB for the historian) as a single --system-prompt
argv token. Windows CreateProcess caps the entire command line at 32,767
characters, so every such spawn failed with ENAMETOOLONG before Pi
started — on Windows the historian never ran at all, and sessions grew
past the context limit unchecked. The existing large-prompt fallback only
rerouted the user message to stdin (calibrated for Linux's 128KB
per-argument limit) and never covered the system prompt.

The runner now writes the system prompt to a per-run temp file and passes
the absolute path (Pi reads an existing path as the replacement prompt
file), on every platform, so the argv-size failure class is gone rather
than special-cased. The file is cleaned up on settle and on spawn
failure. On win32 the user message is additionally always delivered via
stdin, keeping the child command line small regardless of prompt sizes;
other platforms keep the >96KB stdin fallback. buildArgs also passes
--no-context-files so hidden one-shot subagents receive exactly the
system prompt we built instead of having project AGENTS.md/CLAUDE.md
appended by Pi.

Fixes #209

Author: ualtinok