Implement quire exec dispatch with git command allowlist

Implement quire exec dispatch with git command allowlist

Parses SSH_ORIGINAL_COMMAND, validates git commands against an
allowlist (receive-pack, upload-pack, upload-archive), sanitizes
repo paths, and execs the git subprocess. Removes the separate
quire-dispatch shell script — the binary handles dispatch directly.
Updates host reference configs to use quire exec in ForceCommand.

Assisted-by: GLM-5.1 via pi