fix(files,terminal): one path and shell-syntax contract on every OS

[M09Ic]

Summary

• File tools accept either separator, clean . and duplicates, and keep a single / namespace. path.Match treats \ as an escape on every OS, and os.Root wants slash paths.
• Shell scripts are parsed before argv splitting. A literal registered command stays in-process, including quoted empty arguments. Composition runs through a real shell instead of guessing operators from the raw string.
• On Windows the shell is a Win32 bash (Git or MSYS) when one is installed, and cmd.exe only when it is not. Output goes through a pipe in a kill-on-close job, so the console cannot rewrite newlines or trailing spaces and cancel still stops MSYS children that leave the process tree.
• Supersedes fix(commands): route bash scripts using shell syntax #136. The parser and its tests are ported onto the current terminal tool; that branch is not merged on its own.

Absolute workdir joining for a remote runner stays in Cairn. The server is not the runner's OS.

Test plan

• Windows: go test -count=1 ./tools/terminal/ ./tools/files/ ./core/tool/ ./pkg/exts/files/ ./pkg/exts/prompt/
• WSL Ubuntu, same packages
• memory_echo one&&memory_echo two runs both; 'a;b' and a quoted empty argument stay arguments
• Cancel during sleep does not run the rest of the line