v0.3.0

Windows support. Nothing about the harness was portable before this: the budget
hook was a shell script, registrations were shell form, and the guardian only
knew POSIX command shapes.

Changed

• The budget hook is now castra-budget.py. Claude Code runs a shell-form hook
  through sh -c on macOS and Linux but through Git Bash on Windows, falling back
  to PowerShell when Git Bash is absent, so .sh could not work there.
• Hooks register in exec form, spawning the interpreter directly with an
  argument vector and no shell in between. This removes shell selection, quoting
  and variable expansion from the path entirely.
• The installer writes an absolute but deliberately unresolved interpreter path.
  Resolving symlinks pinned a version-specific directory such as
  .../Cellar/python@3.14/3.14.6/..., which disappears on the next patch upgrade
  and takes every hook down without a word.

Added

• install.ps1 for Windows and install.sh for macOS and Linux, both thin
  wrappers over a single install.py so every platform runs identical logic.
  --dry-run reports what would change without writing.
• PowerShell rules in the guardian: recursive Remove-Item, Format-Volume,
  Clear-Disk, Set-ExecutionPolicy, iwr | iex, Start-Process -Verb RunAs,
  service stops, and reading .env with Get-Content.
• tests/run.py, so the suite runs without a shell. CI now runs on Linux, macOS
  and Windows, and installs into a throwaway config directory on each.