Move the SSH exec subcommand interface (help, health, users, stats,
tail, post) and the dispatcher out of ssh_server.c into a dedicated
module.
New API (include/exec.h):
- exec_dispatch(client_t *) -- single entry point invoked from the
bootstrap path when client->exec_command[0] != '\0'.
Helpers that travel with the exec subcommands:
- format_timestamp_utc, trim_ascii_whitespace, json_append_string,
resolve_exec_username, parse_tail_count
Two cross-module bridges:
- is_valid_username() lifted into common.c/h since exec, the input
read path, and the :nick command all need it.
- ssh_server_start_time() added to ssh_server.h as a read-only
accessor; exec_command_stats no longer reaches into the global.
- notify_mentions stays in ssh_server.c for now and is exposed via
ssh_server.h. Will move to a dedicated client.c during PR2-M6.
ssh_server.c shrinks from 2200 to 1769 lines (-431).
Behaviour is preserved: implementations are byte-for-byte the same.
